Question

CxHy completely combusted to produce 344 mg carbon dioxide and 56.2 mg of water. What is its empirical formula?

Answer 1

To determine empirical formula we need to carry out following steps:

Note that for carbon dioxide and water Empirical formula is same as molecular formula.

Step 1. calculate the number of moles for CO₂ and H₂O and then that of each element C and H.

Molecular weight of CO_{2 =} 12 + (2* 16 )= 44.0

Molecular weight of H₂O = (1 * 2) +16 = 18.0

please note that I am using mmoles (this is moles only but smaller)

344 mg of CO₂* (1 mmol of CO₂ / 44.0 mg of CO₂ )= 7.818 mmol CO₂

• 7.818 mmol CO₂ *( 1 mmol of C/ 1 mmol of CO2 ) = 7.818 mmol C

56.2 mg of H₂O * ( 1 mmol of H₂O / 18.0 mg of  H₂O) = 3.122 mmol of  H₂O

• 3.122 mmol of  H₂O * (2 mmol of H / 1 mmol of H₂O ) = 6.244 mmol of H.

Step 2 : Set up a mole ratio by dividing by the smaller moles i.e. divide moles of C and H by moles of H.

7.818 / 6.244 = 1.25 C

6.244/ 6.244 = 1 H

Step 3 : Multiply till the number is whole as empiral formula needs to be in whole numbers.

1.25 * 4 = 5 C

1 * 4 = 4H

So Empirical formula will be C₅H₄