Question

lactic acid is the avid responsible for the sour test in spit milk. on the basis of combustion analysis, it's percentage composition by mass is 40.00% carbon, 6.71%hydrogen and x% oxygen.
a. Calculate the empirical formula of lactic acid
b. The molar mass of lactic acid is 90g/mol. Determine the molecular formula

So I know the first step would be to find the # of g? And then concern that into moles then find the ratio between moles. And then for b you would find the g/mol in the empirical formula and divide it by the molar mass to find how many fit within the molecular formula.

So b makes sense to me, but I'm not sure how to go from the % to the number of g with that little information. Is it just 40g of C and 6.71g of hydrogen, then add those and take away a 100 to find O? That's all I can think of doing, but I don't get why.

Thank you very much

Answer 1

You have it.
Take a 100 g sample which gives you 40 g C, 6.71 g H and (100-40-6.71) g O.
You go from there. Post your work if you get stuck.