Question

When 0.422g of phosphorus is burned, 0.967g of a white oxide is obtained.

Answer 1

Answer :

(a) The empirical formula of a compound is, $P_2O_5$

(b) The balanced chemical equation will be:

$P_4+5O_2\rightarrow 2P_2O_5$

Solution :  Given,

Mass of phosphorus = 0.422 g

Mass of white oxide = 0.967 g

Molar mass of phosphorus = 31 g/mole

Molar mass of oxygen = 16 g/mole

First we have to calculate the mass of oxygen.

Mass of oxygen = Mass of white oxide - Mass of phosphorus

Mass of oxygen = 0.967 - 0.422

Mass of oxygen = 0.545 g

Now convert given masses into moles.

Moles of P = $\frac{\text{ given mass of P}}{\text{ molar mass of P}}= \frac{0.422g}{31g/mole}=0.0136moles$

Moles of O = $\frac{\text{ given mass of O}}{\text{ molar mass of O}}= \frac{0.545g}{16g/mole}=0.0341moles$

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

For P = $\frac{0.0136}{0.0136}=1$

For O = $\frac{0.0341}{0.0136}=2.5$

The ratio of P : O = 1 : 2.5

To make this ratio in a whole number we multiple ratio by 2, we get:

The ratio of P : O = 2 : 5

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

The Empirical formula = $P_2O_5$

The balanced chemical equation will be:

$P_4+5O_2\rightarrow 2P_2O_5$

Answer 2

Moles = mass / molar mass 

moles P = 0.422 g / 30.97 g/mol = 0.01363 mol 
moles O = (0.967 g - 0.422g) / 16.00 g/mol = 0.03406 moles 

So ratio moles P : moles O 
= 0.01363 mol : 0.03406 mol 

Divide each number in the ratio by the smallest number 

(0.01363 / 0.01363) : (0.03406 / 0.01363) 
= 1 : 2.5 

The empirical formula needs to be the smallest whole number ratio of atoms in the molecules. Since you have a non-whole number, multiply the ratio by the smallest number needed to make both number whole numbers. In this case x 2 

2 x (1 : 2.5) 
= 2 : 5