[RenaHay]

When microwave radiation is passed through phosphorus(III) chloride, PCl3 at low pressure, a new chloride phosphorus, 'A', is formed.

'A' contains 69.6% by mass of chlorine and 30.4% by mass of phosphorus, and it's Mr is 200.

Calculate the empirical and molecular formula of 'A'.

Please help me, I don't understand this topic

[Borek]

You have to show your attempts at solving the question to receive help. This is a forum policy.

Hint: if molar mass of a compound is 200, and it contains 30.4% P, how many of those 200 g/mol are from the phosphorus?

Sadly, question is wrong, as molar mass of this particular chloride is not 200, closer to 204.

[RenaHay]

Well, I have the answer

assume 100g of the sample so that the % of each element is also the mass present in the compound, 69.6g (69.6%) is chlorine and 30.4g (30.4%) is phosphorus
69.6g Cl / molar mass Cl = moles Cl
69.6g Cl / 35.45g/mole = 1.96moles Cl (round to 2moles)

30.4g P / molar mass P = moles P
30.4g P / 31g/mole = 0.98moles P (round to 1mole)

molar ratios
moles Cl / moles smallest number = 2Cl / 1P = 2
moles P / moles smallest number = 1P / 1P = 1
PCl2 = 101.9g/mole so this is the empirical formula

200g / 100g = 2
P2Cl4 = molecular formula
2 x 31g/mole + 4 x 35.45g/mole = ~200g

I just don't understand what's going on. Especially at the molar ratios, and what is the 200g? Why must it be divided by 100g? Please help..

[Borek]

No need to divide 200/100. What you did is more or less correct, just slightly convoluted.

If one mole is 200 g/mol, and substance contains 30.4% of phosphorus, 1 mole contains 0.304*200 = 60.8 g of phosphorus, which is almost exactly two moles (1.963 to be exact).

If one mole is 200 g/mol, and substance contains 69.6% of chlorine, 1 mole contains 0.696*200 = 139.2 g of chlorine, which is almost exactly four moles (3.926 to be exact).

One mole of the substance contains 2 moles of P and 4 moles of Cl, so the formula is P₂Cl₄.

Assuming 100 g sample you added one step to your calculations - you had to convert empirical formula (PCl₂) to molecular formula, by using ratio of molecular mass and mass sample (200/100). As you see above this step is not necessary when you know molar mass beforehand.