Question

# 0.36 g of Mg combines with chlorine to produce 1.425 g of magnesium chloride. 9.50 g ofanother sample of anhydrous magnesium chloride gave, on electrolysis, 2.24 litre of chlorine at NTP.Show that these data agree with the law of constant proportions.

Hint: When a compound which is made up of two or more elements has a certain fixed ratio of its
elements no matter from it comes and this is known as the law of constant proportion. So, we have
to prove that the concentration of chlorine is the same from both sources.

- In the given question, we have to prove whether the chlorine is present in the same amount when
different samples were taken.
- As we know that the law of constant proportion states that in the compound, its elements have a
fixed ratio, it does not matter from where they are taken.
- So, it is given that 0.36g of magnesium combines with the chlorine and gives 1.425g of magnesium
chloride.
- The chemical reaction is: $\text{Mg + C}{{\text{l}}_{2}}\text{ }\to \text{ MgC}{{\text{l}}_{2}}$
- So, the mass of the chlorine is $\text{1}\text{.425 - 0}\text{.36 = 1}\text{.065g}$.
- Now, if 1.425g of magnesium chloride is produced 1.065g of chlorine then 1g of magnesium
chloride will be produced $\dfrac{1.0625}{1.425}\text{ = 0}\text{.747g}$ of chlorine.
- So, 9.5g of $\text{MgC}{{\text{l}}_{2}}$ will produce $\dfrac{2.24}{9.5}\text{ = 0}\text{.235L of C}{{\text{l}}_{2}}$.
- Now, the molecular mass of chlorine is 71g which occupies $\text{22}\text{.4d}{{\text{m}}^{3}}$ at
NTP conditions.
- So, $\text{0}\text{.235d}{{\text{m}}^{3}}$ of volume will have $\dfrac{0.235\text{ }\times \text{ 70}\text{.9}}{22.4}\text{ = 0}\text{.744g}$ of chlorine.
- Now, as we can see that the value of the chlorine is close to 1.425g and 9.5g of magnesium
chloride.
Therefore, it follows the law of constant proportion.

Note: NTP stands for the normal temperature and pressure whereas STP is the standar Temperature and pressure. The temperature at NTP and STP is 293.15K and 273.15K respectively. Whereas the value of pressure at NTP and STP is 101 kPa and 100 kPa respectively.