Question

A compound that is composed of carbon, hydrogen, and oxygen contains 70.6% C, 5.9% H, and 23.5% O by mass. The molecular weight of the compound is 136 amu. What is the molecular formula?

Answer 1

C8H8O2

Step-by-step explanation:

simple ratios and division

Answer 2

Answer : The molecular formula of a compound is,
`C_8H_8O_2`

Solution :

If percentage are given then we are taking total mass is 100 grams.

So, the mass of each element is equal to the percentage given.

Mass of C = 70.6 g

Mass of H = 5.9 g

Mass of O = 23.5 g

Molar mass of C = 12 g/mole

Molar mass of H = 1 g/mole

Molar mass of O = 16 g/mole

Step 1 : convert given masses into moles.

Moles of C =
`\frac{\text{ given mass of C}}{\text{ molar mass of C}}= (70.6g)/(12g/mole)=5.88moles`

Moles of H =
`\frac{\text{ given mass of H}}{\text{ molar mass of H}}= (5.9g)/(1g/mole)=5.9moles`

Moles of O =
`\frac{\text{ given mass of O}}{\text{ molar mass of O}}= (23.5g)/(16g/mole)=1.47moles`

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

For C =
`(5.88)/(1.47)=4`

For H =
`(5.9)/(1.47)=4.01\approx 4`

For Cl =
`(1.47)/(1.47)=1`

The ratio of C : H : O = 4 : 4 : 1

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

The Empirical formula =
`C_4H_4O_1=C_4H_4O`

The empirical formula weight = 4(12) + 4(1) + 1(16) = 68 gram/eq

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

Formula used :

`n=\frac{\text{Molecular formula}}{\text{Empirical formula weight}}`

`n=(136)/(68)=2`

Molecular formula =
`(C_4H_4O)_n=(C_4H_4O)_2=C_8H_8O_2`

Therefore, the molecular of the compound is,
`C_8H_8O_2`