Jump to content
Sign in to follow this  

Chemistry past paper questions

Recommended Posts

Hey im just going over some past papers and I just have a few questions I can't answer... Ill just post them here and if someone could help itd be awesome.

2. Which sample has the greatest mass?

A. 6.02 x 10^23 molecules of hydrogen

B. 5.0 mol of neon atoms

C. 1.2 x 10^24 atoms of silver

D. 1.7 x 10^2 g of iron

4. Which sample of nitrogen gas, N2, contains the greatest number of nitrogen molecules?

A. 1.4 g N2

B. 1.4 dm^3 of N2 at 1.01 x 10^5 Pa and 273 K

C. 1.4 x 10^23 N2 molecules

D. 1.4 mol N2

My teacher didn't give me a markscheme so im trying to make one...

Share this post


Link to post
Share on other sites

i don't know if i am correct but i think these questions are testing your understanding of the Avogadro constant especially question 4. When i did the math i got 2. as C and 4 as D. And if i am wrong anybody is free to correct me. You have to remember the formulas they taught you at school like moles= mass/Mr and so on. We should also know for example if we take carbon its molecular mass is 12g or more precisely 12.01 g. What this means is 1 mole of carbon is 12.01g. 1 mole of carbon or 12.01g of carbon equal to 6.02 X 10^23 atoms, this is the avogadro constant as you may know. We should understand this rule in order to the questions. Now 1.01 g of Hydrogen which is the molecular mass of hydrogen in the periodic table is 6.02X 10^23 too. This is because 1 mole of Hydrogen equals 1.01 g. now I will show you how I did the question. Hope my wording isnt confusing:P

Q2.

A. 6.02 X 10^23 → 1 mole of H2 therefore 1.01g = that is the value of A.

B. We can use formula : moles= mass/ mr for this one : 5 moles = mass/ 20.18( mr value in periodic table) Mass= 5 X 20.18 which equals 100.9g.

C. 6.02 X 10^23 atoms → 1 mole of silver therefore 107.87g and then the number of grams for 1.2X10^24 atoms is unknown. So we cross multiply consider the unknown as x.

6.02x10^23→ 107.87g

1.2X10^24→ x

When we cross multiply and make x the subject we get: 107.87X (1.2 X 10^24)/ 6.02 X 10^23 and this equals to 215g to 3 significant figures. So the grams this is 215g

D. Then D is jus simple it 1.7 X 10^2 g which is the same 170g.

Now looking at all the values you found out the biggest mass is evidently 215g. So the answer would be © I dont know if my answers are correct I doubled checked but that is how u do it if I am not wrong. If that was helpful let me know and Ill show you how I did the next question.

Edited by Parami143_Kella

Share this post


Link to post
Share on other sites

Hey im just going over some past papers and I just have a few questions I can't answer... Ill just post them here and if someone could help itd be awesome.

2. Which sample has the greatest mass?

A. 6.02 x 10^23 molecules of hydrogen

B. 5.0 mol of neon atoms

C. 1.2 x 10^24 atoms of silver

D. 1.7 x 10^2 g of iron

4. Which sample of nitrogen gas, N2, contains the greatest number of nitrogen molecules?

A. 1.4 g N2

B. 1.4 dm^3 of N2 at 1.01 x 10^5 Pa and 273 K

C. 1.4 x 10^23 N2 molecules

D. 1.4 mol N2

My teacher didn't give me a markscheme so im trying to make one...

I did stoichiometry in 10th grade so I may be able to help you with the questions.

These are the conversions I usually use.

mass = molar mass x no. of moles

no. of particles = no. of moles x Avagadro's constant (6.023 x 10^23)

Just to let you know, if you want to convert molecules to mass, you have to go through convert the number of particles into number of moles first.

For example:

Let's take 2A. They have given you the number of molecules and you need the mass to answer the question. You convert into moles first by manipulating the equation I gave you above.

mole = 6.02 x 10^23/6.02 x 10^23 = 1mol

mass = mol x 2 (atomic no. of H2) = 2g

If you need more help let us know.

Share this post


Link to post
Share on other sites

Question 4 would be D. In the question you just need to first find the number of moles and then use the formula in the post above.

a) n=1.4/28

=0.05mol

no. of particles=0.05 x 6.02x10^23

=3.01x10^22 molecules

(in b we are given a volume at SLC, so therefore we can use molar volume)

b) n=V/Vm

=1.4/22.4

=0.0625

=0.0625 x 6.02x10^23

=3.7625x10^22 molecules

c)1.4x10^23 molecules

d) n=1.4

no. of particles=1.4 x 6.02x10^23

=8.428x10^23 molecules

So the answer is D.

Share this post


Link to post
Share on other sites

How should one think in question 29? All options seem like combustion to me...

They are asking which ones are incomplete combustions. The first one is complete combustion :P:)

Edited by Parami143_Kella

Share this post


Link to post
Share on other sites

How should one think in question 29? All options seem like combustion to me...

They are asking which ones are incomplete combustions. The first one is complete combustion :P:)

Right. But how can you tell that the first is a complete one?

Share this post


Link to post
Share on other sites

How should one think in question 29? All options seem like combustion to me...

They are asking which ones are incomplete combustions. The first one is complete combustion :P:)

Right. But how can you tell that the first is a complete one?

Ok we have three equations in the question. During complete combustion, there is enough oxygen for the fuel to burn ans as a result carbon dioxide(prodcut)is formed. However, during incomplete combustion there is a lack of oxygen therefore resulting in an inability to form carbon dioxide. In many cases during incomplete combustion carbon monoxide will be formed. Now in the question given, the first equation has formed carbon dioxide but the other two equations haven't. Therefore we can say that the other two is incomplete combustion since carbon dioxide is not formed. Basically just look to see if there is carbon dioxide formed in the products or not if you are given a question like this. Hope that made sense. :)

Share this post


Link to post
Share on other sites

The answer is D for question 17 and the answer is C for question 29. if you could tell which parts you were confused about i will clarify your doubts.

Not to be an ass... but nope, it's C for 17. Reactant particles could collide with insufficient activation energy/wrong geometry and not react.

Right. But how can you tell that the first is a complete one?

Complete combustion: H2O and CO2 produced (sufficient O2)

Incomplete: any of C, CO, CO2, H2O produced (insufficient/limited O2)

Or you could read parami's explanation since she actually explains it, haha.

Share this post


Link to post
Share on other sites

The answer is D for question 17 and the answer is C for question 29. if you could tell which parts you were confused about i will clarify your doubts.

Not to be an ass... but nope, it's C for 17. Reactant particles could collide with insufficient activation energy/wrong geometry and not react.

Sure about this? According to the collision theory, the colliding particles need to, amongst other things

1. Have enough energy to react

2. Collide with the right geometry

And increased frequency of collision (i.e increased concentration) will lead to more collsions. So if I'm correct, all options given are correct, and therefore the answer is D.

So in a complete combustion CO2 is formed? Sounds familiar...

Share this post


Link to post
Share on other sites

The answer is D for question 17 and the answer is C for question 29. if you could tell which parts you were confused about i will clarify your doubts.

Not to be an ass... but nope, it's C for 17. Reactant particles could collide with insufficient activation energy/wrong geometry and not react.

Sure about this? According to the collision theory, the colliding particles need to, amongst other things

1. Have enough energy to react

2. Collide with the right geometry

And increased frequency of collision (i.e increased concentration) will lead to more collsions. So if I'm correct, all options given are correct, and therefore the answer is D.

So in a complete combustion CO2 is formed? Sounds familiar...

Increased collision doesn't guarantee all particles have sufficient energy or the right geometry. Now, provided that these 2 requirements are satisfied, yes, the reaction rate is affected. So it's kind of a trick question... which I'm not too sure of the answer to now because of the wording. Someone check the answer please. :P

Share this post


Link to post
Share on other sites

The answer is D for question 17 and the answer is C for question 29. if you could tell which parts you were confused about i will clarify your doubts.

Not to be an ass... but nope, it's C for 17. Reactant particles could collide with insufficient activation energy/wrong geometry and not react.

Sure about this? According to the collision theory, the colliding particles need to, amongst other things

1. Have enough energy to react

2. Collide with the right geometry

And increased frequency of collision (i.e increased concentration) will lead to more collsions. So if I'm correct, all options given are correct, and therefore the answer is D.

So in a complete combustion CO2 is formed? Sounds familiar...

Increased collision doesn't guarantee all particles have sufficient energy or the right geometry. Now, provided that these 2 requirements are satisfied, yes, the reaction rate is affected. So it's kind of a trick question... which I'm not too sure of the answer to now because of the wording. Someone check the answer please. :D

But increased frequency will increase the rate of reaction. It doesn't matter how high energy they have, if they don't meet, they won't react.

Share this post


Link to post
Share on other sites

The answer is D for question 17 and the answer is C for question 29. if you could tell which parts you were confused about i will clarify your doubts.

Not to be an ass... but nope, it's C for 17. Reactant particles could collide with insufficient activation energy/wrong geometry and not react.

Sure about this? According to the collision theory, the colliding particles need to, amongst other things

1. Have enough energy to react

2. Collide with the right geometry

And increased frequency of collision (i.e increased concentration) will lead to more collsions. So if I'm correct, all options given are correct, and therefore the answer is D.

So in a complete combustion CO2 is formed? Sounds familiar...

Increased collision doesn't guarantee all particles have sufficient energy or the right geometry. Now, provided that these 2 requirements are satisfied, yes, the reaction rate is affected. So it's kind of a trick question... which I'm not too sure of the answer to now because of the wording. Someone check the answer please. :D

But increased frequency will increase the rate of reaction. It doesn't matter how high energy they have, if they don't meet, they won't react.

There will always be collisions between particles, that's a given. I'm not arguing this, haha. Someone check the answer, please.

Share this post


Link to post
Share on other sites

The answer is D for question 17 and the answer is C for question 29. if you could tell which parts you were confused about i will clarify your doubts.

Not to be an ass... but nope, it's C for 17. Reactant particles could collide with insufficient activation energy/wrong geometry and not react.

Sure about this? According to the collision theory, the colliding particles need to, amongst other things

1. Have enough energy to react

2. Collide with the right geometry

And increased frequency of collision (i.e increased concentration) will lead to more collsions. So if I'm correct, all options given are correct, and therefore the answer is D.

So in a complete combustion CO2 is formed? Sounds familiar...

Increased collision doesn't guarantee all particles have sufficient energy or the right geometry. Now, provided that these 2 requirements are satisfied, yes, the reaction rate is affected. So it's kind of a trick question... which I'm not too sure of the answer to now because of the wording. Someone check the answer please. :D

But increased frequency will increase the rate of reaction. It doesn't matter how high energy they have, if they don't meet, they won't react.

There will always be collisions between particles, that's a given. I'm not arguing this, haha. Someone check the answer, please.

:D From what i know, heat increases the collision frequency and hence collision frequncy does affect the rate of reaction. However, like Irene said, for a collision to occur it requires energy and the correct geometry. However, the question does not state if the reaction has already started or not. I think in this question the collision frequency does affect the rate of reaction. But someone should check the answer:) i will look for it. And yes Tilia CO2 is produced in complete combustion and in incomplete it is usually CO or just C. :)

Share this post


Link to post
Share on other sites

From what i know, heat increases the collision frequency and hence collision frequncy does affect the rate of reaction. However, like Irene said, for a collision to occur it requires energy and the correct geometry. However, the question does not state if the reaction has already started or not. I think in this question the collision frequency does affect the rate of reaction. But someone should check the answer:) i will look for it. And yes Tilia CO2 is produced in complete combustion and in incomplete it is usually CO or just C. :P

Be very careful with that first statement.

Heat increases the number of particles with the required activation energy - this is the reason why the reaction is faster. The IBO will not give you the mark for saying that there are more collisions as a reason for temperature increasing the rate (it does, but this is a minor factor).

However, if you are asked about collision frequency specifically then, yes, increased collisions increases the rate of reaction. This is why concentration increase speeds up the reaction, there are more collisions overall(and hence more with the correct orientation and activation energy)

It is helpful to learn to relate this topic with the Maxwell-Boltzmann curve as it provides most of the detail required (not orientation).

In summary

---------

Increase the temperature

1. the no of particles with the required activation energy increases - more (greater number) of effective collisions

2. The number of collisions also increases (minor factor)

Increase concentration (or pressure of gas)

1. The number of collisions increases and hence the number of effective collisions

Add a catalyst

1. The energy required for effective collsion is lower, therefore there are more (greater number) effective collisions

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
Sign in to follow this  

×

Important Information

We have placed cookies on your device to help make this website better. You can adjust your cookie settings, otherwise we'll assume you're okay to continue.