Fall 2015 Quiz Prep question #5 [ENDORSED]

[Kim]

Can someone explain this to me? Sorry if this has already been answered elsewhere but I couldn't find it

"6.4g of a compound was burned in air and produced 8.80g CO2 and 7.20g H2O. Find the empirical formula of the compound."

the answer is CH4O

[Jenna_Hakel_2A]

So to find the empirical formula, you first find the number of moles of Carbon and Hydrogen given the masses produced of carbon dioxide and water. The tricky part in this question is that it is burned in air, so O2 can also contribute to the oxygen in the products; it is not only from the reactant that we are trying to solve for. So, to find the number of moles of oxygen that is from the compound, convert the number of moles of hydrogen and carbon that you found back to grams and subtract the total of these 2 masses from the mass of the compound. So 6.4-(mass of hydrogen+mass of carbon). This will give you the mass of the oxygen that is from the compound instead of from both the compound and the oxygen in the air. Given this mass, determine number of moles of oxygen that are from the compound. So now, you should have the total number of moles of all of the different elements in the compound, divide by the smallest value to find the ratio, and you have an empirical formula! :)

[Michelle_Li_1H]

Hi,

Regarding the ratio, I got 1:4.5:1 for C, H, and O respectively. However, I was wondering how you would know to round down to 4 for H instead of multiplying the entire thing by 2 to get a whole number for H? Thanks!

[Abigail Foster 1K]

In order to get the ratio, you have to divide all the moles by the smallest number in-between them (0.2 mol O, 0.2 mol C, and 0.8 mol H, so you would divide all the numbers by 0.2), once you divide you round to the nearest whole number. After, there you have it your empirical formula.