Molecular Formuals from Empirical Formula

[HigueraC]

Homework Statement

An unknown compound contains 21.8 g of phosphorus and 28.2 g of oxygen. It's molar mass is 284 g/mol. what are its empirical and molecular formulas?

Elements and the Weight:
P = 31 g/mol
O = 16 g/mol

Given Measurements:
21.8g P, 28.2g O, MM = 284g/mol



2. The attempt at a solution

This is my math:
Elements and the Weight:
P = 31 g/mol
O = 16 g/mol

Given Measurements:
21.8g P, 28.2g O, MM = 284g/mol

Converting the given measurements to moles:
21.8gP / 31 = .70 mol P
28.2gO / 16 = 1.8 mol O

Getting the ratio:
.70 / .70 = 1
1.8 / .70 = 2.57 (rounds up to 3)

Ratio:
1:3

Empirical Formula : PO3

Formula Mass of PO3:
79g/mol

Finding the common factor:
284(the given weight)/ 79(the weight of PO3) = 3.594 (rounds up to 4)

Writing the Molecular Formula:
4(PO3) = P4O12

Getting the Molar Mass of the Molecular Formula:
P4 = 124
O12 = 192
P4O12 = 316
316 =/= 284(is what it should be)

 

[HigueraC]

Nevermind, I did some research and found out that if the ratio is ~2.5 that you're supposed to multiply it by 2 to get even numbers.

 

[Tom Creswell]

Homework Statement

An unknown compound contains 21.8 g of phosphorus and 28.2 g of oxygen. It's molar mass is 284 g/mol. what are its empirical and molecular formulas?

Elements and the Weight:
P = 31 g/mol
O = 16 g/mol

Given Measurements:
21.8g P, 28.2g O, MM = 284g/mol



2. The attempt at a solution

This is my math:
Elements and the Weight:
P = 31 g/mol
O = 16 g/mol

Given Measurements:
21.8g P, 28.2g O, MM = 284g/mol

Converting the given measurements to moles:
21.8gP / 31 = .70 mol P
28.2gO / 16 = 1.8 mol O

Getting the ratio:
.70 / .70 = 1
1.8 / .70 = 2.57 (rounds up to 3)

Ratio:
1:3

Empirical Formula : PO3

Formula Mass of PO3:
79g/mol

Finding the common factor:
284(the given weight)/ 79(the weight of PO3) = 3.594 (rounds up to 4)

Writing the Molecular Formula:
4(PO3) = P4O12

Getting the Molar Mass of the Molecular Formula:
P4 = 124
O12 = 192
P4O12 = 316
316 =/= 284(is what it should be)

====================
You got a ratio of 2.57:1. instead of rounding to 3, try 5:2, which is a lot closer.
Then your formula is P2O5 or P4O10. This makes sense since P and O have oxidation numbers of +5 and -2, so the electrons balance out to zero.