Question

Determine the empirical formula for a compound that is found to contain 10.15 mg P and 34.85 mg Cl.?

But I'm confused because I thought I had to convert 10.15 mg to grams but then I get a huge number if I do that. Why do I leave it as mg?

Answer 1

To convert the amounts into grams, we divide by 1000.
P = 0.01015 g
Cl = 0.03485 g

We now divide each amount by their atomic masses to find the moles:

moles(P) = 0.01015 / 31 = 3.39 x 10⁻⁴
moles(Cl) = 0.03485 / 35 = 9.96 x 10⁻⁴

We now divide by the smaller number to find the empirical forumla:
P = 1
Cl = 9.96/3.39 ≈ 3

Formula:
PCl₃