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AChapel

Significant Figures

How do you know how many significant figures to use in an example like this:

12.3gO x 1 mol/16.00gO = X

There is only one significant figure in the moles (one) so do I use that or do I go by the actual numbers?

Thanks!

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How do you know how many significant figures to use in an example like this:

12.3gO x 1 mol/16.00gO = X

There is only one significant figure in the moles (one) so do I use that or do I go by the actual numbers?

Thanks!

You have to consider the decimal points. The maximum number decimal place, is 1 as in 12.3. So change everything to 1 decimal digit. so you get 12.3g x 1.0 mol/16.0g = 0.8 mol

Edited by Solaris

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You have to consider the decimal points. The maximum number decimal place, is 1 as in 12.3. So change everything to 1 decimal digit. so you get 12.3g x 1.0 mol/16.0g = 0.8 mol

Are you sure? I thought that you only consider decimal places when it comes to addition or substraction but for multiplication and division you consider the least number of significant figures. Can anyone confirm this?

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Oh, sorry. Just Confirmed it. It only applies to addition and subtraction.

Multiplication and Division

When experimental quantities are multiplied or divided, the number of significant figures in the result is the same as that in the quantity with the smallest number of significant figures. If, for example, a density calculation is made in which 25.624 grams is divided by 25 mL, the density should be reported as 1.0 g/mL, not as 1.0000 g/mL or 1.000 g/mL.

Edited by Solaris

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How do you know how many significant figures to use in an example like this:

12.3gO x 1 mol/16.00gO = X

There is only one significant figure in the moles (one) so do I use that or do I go by the actual numbers?

Thanks!

The units are g and g/mol, where mol is exactly 1 mol (so 1.00000000 mol etc). Therefore, there are 3 sig digs.

12.3g / 16.00g/mol = 0.769mol

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The units are g and g/mol, where mol is exactly 1 mol (so 1.00000000 mol etc). Therefore, there are 3 sig digs.

12.3g / 16.00g/mol = 0.769mol

I think it should be the lowest number of significant digits in the calculations, which makes it:

12.3g / 16 g/mol = 0.77 mol

The maximum number of significant figures is 2 (as in 16).

Edited by Solaris

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I think it should be the lowest number of significant digits in the calculations, which makes it:

12.3g / 16 g/mol = 0.77 mol

The maximum number of significant figures is 2 (as in 16).

No, the trailing zeros in the 16.00 are there for a reason. You can't just take them off! So the person above you is right the least number of significant figures is 3.

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No, the trailing zeros in the 16.00 are there for a reason. You can't just take them off! So the person above you is right the least number of significant figures is 3.

Uh, ok. But if you are right than we also need to change 12.3 to 12.30

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Uh, ok. But if you are right than we also need to change 12.3 to 12.30

No, that's the way they gave it to you 12.3 and 16.00 so you can't add or take off digits. It's a given that 12.3 has 3 sig. fig. and 16.00 has 4.

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sig fig rules for addition and subtraction:

smallest sig fig.

ex) 24 + 20 = 26 = 30

sig fig rule for multiplication and division:

smallest decimal point

ex) 2.5 x 3 = 7.5 = 8

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sig fig rules for addition and subtraction:

smallest sig fig.

ex) 24 + 20 = 26 = 30

sig fig rule for multiplication and division:

smallest decimal point

ex) 2.5 x 3 = 7.5 = 8

Incorrect, actually. -_-

Addition and subtraction:

keep the least number of decimal places

ex) 1.38 + 3.93 + 2.2 = 7.5

Multiplication and division:

keep the least number of significant digits

ex) 2.593 * 94.2 / 3.1930 = 74.5

Counting the number of sig digs

Trailing zeroes AFTER the decimal place count

ex) 3.00 has 3 sig digs

Trailing zeroes before the decimal place do NOT count UNLESS there is a decimal place

3200 has 2 sig digs

320. has 3 sig digs

320.0 has 4 sig digs

Leading zeroes do not count

ex) 0.0000291 has 3 sig digs

0.003200 has 4 sig digs

004 has 1 sig dig

Zeroes between numbers DO count

ex) 605 has 3 sig digs

All non-zero numbers count

ex) 483 has 3 sig digs

SCIENTIFIC NOTATION is often useful when calculating with sig digs

ex) 0.0003200 has 4 sig digs

converted to 3 sig digs = 3.20 x 10-4

converted to 2 sig digs = 3.2 x 10-4

ex) 30000 has 1 sig dig

converted to 2 sig digs = 3.0 x 104

converted to 3 sig digs = 3.00 x 104

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