[bholley]

I used to know how to do this, but I can't remember how to get started. These aren't the actual numbers for my problem - I just want to learn how to do it.

You are given three compounds that consist of two elements (X,Y, and/or Z) and decompose them to their respective elements. To determine the relative masses of X, Y, and Z, you collect and weigh the elements, obtaining the following data:

Elements in compound     Masses of elements
1. X and Y                        X=0.4g, Y=4.2g
2. Y and Z                        Y=1.4g, Z=1.0g
3. X and Y                        X=2.0g, Y=7.0g

What are the assumptions needed to solve this problem (X₂Y and such, I believe), What are the relative masses of X, Y, and Z (does that mean what are the elements?) and what are the chemical formulas of the three compounds?

Thanks to anyone who helps me out. It's not extremely urgent as far as assignments go, but it bugs me that I can't figure it out.

[Borek]

Convert masses to moles, find molar ratios.

[bholley]

Yeah, that's what I'd do, but I don't know what the elements are, so I don't have that conversion factor. Do I mess around with Avogadro's number? I don't see that getting me anywhere either, though.

[Borek]

Sorry, I misread the question in a hurry, my mistake.

This has to be done by trial and error. First and last compound are made of the same elements. Compare mass ratios, can you spot something interesting?

Relative masses mean - assume molar mass of one of these elements to be 1 (or any other number) and express molar masses of other elements as ratio of their molar masses to the molar mass of the standard.

[bholley]

I'm still not following, sorry. Which mass ratios should I take? X from experiment 1 to X of 2? Nothing really stands out. I'll try to ask my professor tomorrow if he has time to show me how.

The answers are in multiple multiple choice form, as in:
the formula in experiment 1 is XY,
the formula in experiment 2 is YZ,
the formula in experiment 3 is X₃Y
the formula in experiment 3 is XY₃
the formula in experiment 1 is XY₂

Those are assumptions and I'm supposed to pick which ones are needed to solve the problem.

[Borek]

Start with ratio of masses of elements in each compound. See if the ratio of ratios is random, or is it close to some integer. If it is - does it tell something about possible formulas?