Asked by Amy~

For this problem would I have to use proportions?

Analysis of a metal chloride XCI3 shows that it contains 67.2 % CI by mass. Calculate the molar mass of X and identify the element.

I tried using proportions:
X+CI3/CI3 = ?/67.2
but then I had to find for both X and ?. The question doesn't give the mass of XCI3.
Answered by Amy~
The answers were 51.8g and element Cr
Answered by MathMate
The denominator is 67.2%, means that you can use 100 for the numerator and 67.2 for the denominator.

Proceed the same way as the other problem (Zinc) and post your answer if you are not sure.
Answered by Amy~
I did:
X + CI3/ CI3 = 100/ 67.2

67.2(x+CI3) = 100 CI3

67.2x = 32.8 CI3

x = 0.488 X 100 = 48.8% or grams

Then would I subtract 48.8 from 100 to get 51.2g (would I subtract because I solved for X)?
Answered by MathMate
From 67.2x = 32.8 CI3
you would substitute Cl3 by 3*35.457 (3*atomic mass of chlorine) and solve for x:
x=32.8*35.457/67.2
= 51.92

Look up the periodic table to find the element that has the closest atomic mass.
Chromium is 51.996 compared to 51.92 gives an error of 0.15%.
Answered by Amy~
ok, thank you
Answered by MathMate
You're welcome.
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