CHEMISTRY COMMUNITY

When calculating for the empirical formula, you must first calculate for the limiting reactant. The part I am confused on is how do you know what number to multiply by when your number isn't whole?
For example, if you have 100 grams of H2O and you divide by 18g/mol of H2O (the molar mass) your final answer would be 11.1 mol, assuming that the limiting reactant in this problem was 1.56 mol, you would then divide 11.1/1.56 which would be 7.30, how do I know what number to multiply it in order to get a whole number? Can I just round down to 7? When is it safe to round down or up?

Sorry if this is worded funny.

Well it is much closer to 7.33 than 7 so I would round to 7.33 and then multiply it by 3 to get a whole number.

Typically you don't want to round by more than 0.05, so I recommend rounding something like 7.30 to 7.33 and then multiply by 3 to get 22. When working with the empirical formula in particular, if you get numbers that are 0.1 or less off from a whole number then usually you are in the clear, but it is always useful to be careful and try multiplying by a couple other numbers if you are worried to make sure there is no closer you can get to a whole number. You won't often get numbers that multiply absolutely perfectly to a whole number, so just use your judgement to know if something is close enough to round. Like I said before, rounding 5.95 to 6 is usually acceptable, but I would not recommend rounding by anything greater than .1

Hope this helps!

I see that your question has been pretty much answered but I'd recommend using the limiting rule of on no more than .1 to check your work. If it takes you more than a few calculation to get to a whole number of moles it is clear something went wrong. I tend to make a lot of little mistakes so it helps for me to check my work as often as possible. I hope this helps!

When I do limiting reactant/empirical formula problems I usually find the ratios then instead of multiplying to a whole number, I first look to see if it converts to some nice fraction-- eg 4.33 = 13/3 or 4.63-4.68 = 14/3, then solve from there. :)

Just like everyone else has already said, it's usually not recommended to round up or down by more than 0.1. So in your case, you would not want to round down 7.30 to 7. Everyone uses their own method to determine what factor to multiply their moles by, but personally I like to use a simple method of guess and check. Since you want to multiply by the smallest factor possible, I just start by multiplying the digit by 2, 3, 4, and so on, until I reach a number that is either a whole number or can be easily rounded off to a whole number. For your example, I would first do 7.30 x 2 = 14.6 (Since this would require us to round by more than 0.1, I would not use a multiple of 2). Next, I would do 7.30 x 3 = 21.9 (Since we can round up by 0.1 and get 22, we can use 3 as our multiple!) I hope this helps :)