[Dodofish]

This question is to do with calculation of molecular masses in general but I thought I'd use something simple (eg acetone) to describe my query.

Acetone has 3 carbons, 1 oxygen and 6 hydrogens; by my reckoning this should make its molecular mass 58.08 g/mol:

3*12.01 + 16.00 + 6*1.008 = 58.08

However, looking online, a large number of search results suggest 58.09 g/mol is the molecular mass. I presume that this is using an atomic mass for H with 3 significant figures:

3*12.01 + 16.00 + 6*1.01 = 58.09

I'd like to know which is the right answer (and whether the way I calculated the answer is incorrect)

Is it incorrect to simply add 6*1.008 to the other components in the first equation, given that they have different numbers of decimal places, or is it fine to add all the terms and then worry about rounding at the end?

Cheers

[Bakegaku]

The method you used is correct, but I'd say that you should probably use the same number significant figures in both numbers (the atomic mass of carbon and that for hydrogen) so you should probably use 12.01 and 1.01.

[Dodofish]

Do you mean the same number of decimal places? 12.01 and 1.01 don't have the same number of significant figures.

I'd like to know what the mass of acetone is correct to 4 significant figures, i.e. what would it be if it was determined experimentally.

Surely one of the masses I mentioned (58.08 g/mol or 58.09 g/mol) has to be incorrect.

It may seem picky to differentiate between the two, as I'd imagine for most practical applications the final significant figure will be made redundant by error, but I'd like to know the correct way of doing it.

[xiankai]

for measured values, round off to the lowest number of decimal places among the numbers added.

for calculated values, round off to the lowest number of significant figures among the numbers added.

i also think your method is correct, to round off at the end.

while the value of 58.08 g/mol can be obtained by summing up the molecular masses, it seems to be that the value of 58.09 g/mol could have been obtained experimentally, because it only has 4 significant figures as opposed to 6 significant figures for the Mr of C, O and H. so your value of 58.08 g/mol should be more accurate and correct.