Question

A sample of a compound contains 60.0 g C and 5.05 g H. Its molar mass is 78.12 g/mol. What is the compound’s molecular formula?

Answer 1

check it out. Solved it.

Answer 2

Answer : The molecular formula of the compound is, $C_6H_6$

Solution : Given,

Mass of C = 60 g

Mass of H = 5.05 g

Molar mass of C = 12 g/mole

Molar mass of H = 1 g/mole

Step 1 : convert given masses into moles.

Moles of C = $\frac{\text{ given mass of C}}{\text{ molar mass of C}}= \frac{60g}{12g/mole}=5moles$

Moles of H = $\frac{\text{ given mass of H}}{\text{ molar mass of H}}= \frac{5.05g}{1g/mole}=5.05moles$

Step 2 : For the mole ratio, divide each value of moles by the smallest number of moles calculated.

For C = $\frac{5}{5}=1$

For H = $\frac{5.05}{5}=1.01\approx 1$

The ratio of C : H = 1 : 1

The mole ratio of the element is represented by subscripts in empirical formula.

The Empirical formula = $C_1H_1$

The empirical mass of $C_1H_1$  = 12 + 1 = 13 g/mole

The given molar mass of compound = 78.12 g/mole

Now we have to calculate the molecular formula of the compound.

Formula used : $n=\frac{\text{Molar mass}}{\text{Empirical mass}}$

where, n is valency

$n=\frac{\text{Molar mass}}{\text{Empirical mass}}=\frac{78.12}{13}=6.009\approx 6$

Now the number of valency is, 6

The molecular formula of the compound is, $C_6H_6$

Therefore, the molecular formula of the compound is, $C_6H_6$