Find the empirical formula

Question

A compound containing carbon and hydrogen contains 4.75g of carbon and 0.80g of hydrogen. The molar mass of the compound is about 28g/mol.

1. What is the empirical formula?

Answer 1

Step1: First step to determine the mass% of C and H.

Mass % of carbon in compound = Mass of carbon/Total mass of compound.

Mass% of Carbon =(4.75/4.75+0.80)=85.58%.

Mass% of H =100-Mass% of C.

Mass% of H =100-85.58=14.42%.

Step 2: Divide the percents by the molar masses to get the moles

C: 85.58/12=7.3 moles

H: 14.42/1= 14.42 moles

Step 3: Divide both numbers with the smaller moles

C: 7.3/7.3 =1

H: 14.42/7.3= 2.02

Empirical formula is CH2

Answer 2

The starting point on this one is converting Mass to Moles using the Molar Masses

Moles of C: 4.75 g / 12.01 g/mol = 0.3955 mol

Moles of H: 0.80 g / 1.008 g/mol = 0.7937 mol

Meaning that the ratio in the Empirical Formula is 1 mol C to 2 mol H

Finally then you want to see if those values need to be scaled at all for the Molecular Formula

Since we're given that the total molar mass of the compound is 28, you want to take

28 g/mol / 14.026 g/mol = 2

So we want to scale each value in the Empirical Formula by 2 to get to the Molecular Formula

(CH₂)*2 = C₂H₄

Answer 3

C H

mass= 4.75g 0.80 g

moles= 4.75 g/12 g mol^-1 0.80g/ 1gmol^-1

moles= 0.396 mol 0.80 mol

mole ratio= 0.396mol/0.396mol 0.80 mol/0.396 mol

mole ratio= 1 2

empirical formula = CH₂

Answer 4

No need to go through the extra step of finding mass %. You can simply determine moles of C and moles of H.

moles of C = 4.75 g C x 1 mol C/12 g = 0.396 moles C

moles of H = 0.80 g H x 1 mol H/1 g = 0.80 moles H

Dividing by 0.396 to get whole numbers we end up with..

0.396/0.396 = 1 for C

0.80/0.396 = 2 for H

Empirical formula = CH2