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IB Chemistry Revision Questions - Quantitative Chemistry, Topic 1

Give these questions a go :clap:

Answers tomorrow ......

In the meantime, let me know if you have any problems :surrender:

1, Which of the following has the greatest mass?

A. 6 x 1025 atoms of helium gas

B. 10 moles of oxygen molecules

C. 1.2 x 1024 atoms of copper

D. 1 mole of gold atoms

2, Barium peroxide, BaO2, decomposes when heated to give BaO and O2.

2BaO2 --> 2BaO + O2

If 0.500 mol of BaO2 is decomposed, the number of moles of O2 formed is

A. 0.250

B. 0.500

C. 1.00

D. 4.00

3, Which of the following is an empirical formula?

A. Al2Cl6

B. H2C2O4

C. C2H6

D. CH4O

4, How many grams of AgCl would be precipitated if an excess of AgNO3 solution were added to 55.0 cm3 of 0.200 M KCl solution?

AgNO3(aq) + KCl(aq) --> KNO3(aq) + AgCl(s)

5, A Household cleaners contains ammonia. 25.37g of this cleaner is dissolved in water and made up to 250 cm3. A 25.0 cm3 portion of this solution requires 37.3 cm3 of 0.360 mol/dm3 sulphuric acid for neutralisation.

What is the percentage by mass of ammonia in the cleaner ?

2NH3(aq) + H2SO4(aq) --> (NH4)2SO4 (aq)

6, Hydrogen sulphide burns in oxygen according to the following equation

2H2S(g) + 3O2(g) --> 2H2O(g) + 2SO2(g)

If 4dm3 of H2S are burned in 10 dm3 of oxygen, what is the final volume of the mixture? (assume that all volumes are measured at the same temperature and pressure).

7, An organic compound has the composition by mass of 83.5% carbon, 6.4% hydrogen and 10.1% oxygen. The molar mass of the compound is 158 g mol-1. Determine the molecular formula of the compound.

8, A 25.0 g sample of an oxide of copper, when heated in a stream of hydrogen, forms 3.15 g of water. Find the percentage of copper by mass in the compound.

Hint: The Hydrogen removes the copper from the water.

CuO + H2 --> Cu + H2O

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Hi candy6,

Here are my answers - take a look and let me know if there is anything you are unsure of:

IB Chemistry Revision Questions - Quantitative Chemistry, Topic 1

Answers!

1 = A

2 = A

3 = D

4:

0.2 x (55/1000) = 0.011 mol of KCl

Which also = 0.011 mol of AgNO3

Mass = mol x Mr = 0.011 x 143 = 1.573g

5:

Moles H2SO4 = 0.360 x (37.3/1000) = 1.332 x 10 -2 mol H2SO4 in 25cm3

Moles of NH3 = 2 x 1.332 x 10 -2 = 2.664 x 10 -2 mol in 25cm3

In 250 cm3 = 2.664 x 10 – 1 mol NH3

Mass NH3 = 0.2664 x 17 = 4.57g

% mass = (4.57 / 25.37) x 100 = 18.0%

6:

4dm3 H2S needs 6dm3 of O2

Therefore O2 is in excess.

H2S = limiting reagent

All H2S react to produce 4dm3 H2O and 4dm3 SO2

8 dm3 of products plus 10 -6 = 4 dm3 of O2 left over

So 12 dm3 of final mixture

7:

Carbon 83.5 / 12 = 6.96

Hydrogen 6.4 / 1 = 6.4

Oxygen 10.1 / 16 = 0.63

There froe divide all answers by 0.63

Carbon = 6.96 / 0.63 = 11

Hydrogen = 6.4 / 0.63 = 10

Oxygen = 0.63 / 0.63 = 1

Empirical formula = C11H10O

8:

Moles H2O = 3.15 / 18 = 0.175 mol

This also = 0.175 mol O in compound

Mass of O = 0.175 x 16 = 2.8g

Therefore, mass of Cu in compound = 25.0 = 2.8 = 22.2g

% by mass = (22.8 / 25) x 100 = 88.8%

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5:

Moles H2SO4 = 0.0360 x (37.3/1000) = 1.332 x 10 -3 mol H2SO4 in 25cm3

Moles of NH3 = 2 x 1.332 x 10 -2 = 2.664 x 10 -2 mol in 25cm3

In 250 cm3 = 2.664 x 10 – 1 mol NH3

Mass NH3 = 02664 x 17 = 4.57g

% mass = (4.57 / 25.37) x 100 = 18.0%

I'm quite confused... The question states 0.360 mol/dm3. Is this just a typo? Moreover, when you're trying to find the moles of NH3, why did 1.332 x 10-3 turn into 1.332 x 10-2 mol?

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Hi ilovepotato,

You are correct - there were a few typos in this answer, although the overall answer is correct - apologies!

Here is the corrected answer:

Moles H2SO4 = 0.360 x (37.3/1000) = 1.332 x 10 -2 mol H2SO4 in 25cm3

Moles of NH3 = 2 x 1.332 x 10 -2 = 2.664 x 10 -2 mol in 25cm3

In 250 cm3 = 2.664 x 10 – 1 mol NH3

Mass NH3 = 0.2664 x 17 = 4.57g

% mass = (4.57 / 25.37) x 100 = 18.0%

Does this make sense?

Have you tried today's problems?

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5:

Moles H2SO4 = 0.360 x (37.3/1000) = 1.332 x 10 -2 mol H2SO4 in 25cm3

Moles of NH3 = 2 x 1.332 x 10 -2 = 2.664 x 10 -2 mol in 25cm3

In 250 cm3 = 2.664 x 10 – 1 mol NH3

Mass NH3 = 0.2664 x 17 = 4.57g

% mass = (4.57 / 25.37) x 100 = 18.0%

Hi,

Thanks for the questions, but I don't understand where you get the 1.332x10-2 from (Moles of NH3). I get why you multiply by 2 (molar ratio, right?)

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Hi BioNinja,

What did you get for the moles of NH3?

Did you get 0.01322?

If so, this is the same as 1.322 x 10 -2!

Does that make sense?

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Ohhh wow that's pretty obvious :) just a dumb moment I guess. Yeah that's perfect, thanks !

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