Question

Determine the molecular formula for each compound.

Answer 1

The molecular formula for compound (a) is H₂O₂

The molecular formula for compound (a) is C₆H₆O₄

Explanation:

A compound's molar mass determines what is the mass of one mole of that substance. One mole of a compound contains Avogadro's number of molecules = $6.022\times 10^\ 23$ molecules.

The percent composition of elements in 1 mole of compound can be used to get the exact number of moles of each element present in per mole of compound.

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a) 94.1% Oxygen and 5.9% Hydrogen; molar mass = 34 g

The given compound's percent composition is as follows :

$Oxygen = 94.1\% \\Hydrogen = 5.9\%$

For O :

94.1% of oxygen refers to 94.1 g of Oxygen per 100 g of compound.

Mass of Oxygen in 34 g of compound = $34\times \frac{94.1}{100} = 31.994\ g$

To determine the number of moles of element O present in one mole of compound :

Molar mass of Oxygen = 15.9994 g/mol

The mass in grams of one mole of substance is called molar mass.

One mole of oxygen has 15.9994 grams of O.

Number of moles of O in 31.994 g of O = $\frac{31.994}{15.9994} = 1.9996$

In whole numbers,

Number of moles of O in 31.994 g of compound = 1.9996 ≈ 2

Similarly,

For H :

5.9% of hydrogen refers to 5.9 g of Hydrogen per 100 g of compound.

Mass of Hydrogen in 34 g of compound = $34\times \frac{5.9}{100} = 2.006\ g$

To determine the number of moles of element H present in one mole of compound :

Molar mass of Hydrogen = 1.00794 g/mol

The mass in grams of one mole of substance is called molar mass.

One mole of hydrogen has 1.00794 grams of H.

Number of moles of H in 2.006 g of H = $\frac{2.006}{1.00794} = 1.9901$

In whole numbers,

Number of moles of H in 2.006 g of compound = 1.9901 ≈ 2

The molecular formula for this compound (a) can be written as $H_2O_2$

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b) 50.7% Carbon, 4.2% Hydrogen, and 45.1% Oxygen; molar mass = 142 g

The given compound's percent composition is as follows :

$Carbon = 50.7\%\\Hydrogen = 4.2\%\\Oxygen = 45.1\%$

For C :

50.7% of carbon refers to 50.7 g of carbon per 100 g of compound.

Mass of Carbon in 142 g of compound = $142\times \frac{50.7}{100} = 71.994\ g$

To determine the number of moles of element C present in one mole of compound :

Molar mass of Carbon = 12.011 g/mol

The mass in grams of one mole of substance is called molar mass.

One mole of carbon has 12.011 grams of C.

Number of moles of C in 71.994 g of C = $\frac{71.994}{12.011} = 5.9940$

In whole numbers,

Number of moles of C in 71.994 g of compound = 5.9940 ≈ 6

For H :

4.2% of hydrogen refers to 4.2 g of Hydrogen per 100 g of compound.

Mass of Hydrogen in 142 g of compound = $142\times \frac{4.2}{100} = 5.964\ g$

To determine the number of moles of element H present in one mole of compound :

Molar mass of Hydrogen = 1.00794 g/mol

The mass in grams of one mole of substance is called molar mass.

One mole of hydrogen has 1.00794 grams of H.

Number of moles of H in 5.964 g of H = $\frac{5.964}{1.00794} = 5.9170$

In whole numbers,

Number of moles of H in 2.006 g of compound = 5.9170 ≈ 6

For O :

45.1% of oxygen refers to 45.1 g of Oxygen per 100 g of compound.

Mass of Oxygen in 142 g of compound = $142\times \frac{45.1}{100} = 64.042\ g$

To determine the number of moles of element O present in one mole of compound :

Molar mass of Oxygen = 15.9994 g/mol

The mass in grams of one mole of substance is called molar mass.

One mole of oxygen has 15.9994 grams of O.

Number of moles of O in 64.042 g of O = $\frac{64.042}{15.9994} = 4.002$

In whole numbers,

Number of moles of O in 31.994 g of compound = 4.002 ≈ 4

The molecular formula for this compound (b) can be written as $C_6H_6O_4$