Question

The weight of hydrogen gas obtained from $42g$ of $Ca{{H}_{2}}$ by treatment with water is --- gm.

Answer 1

Hint: In this, first of all we will write the reaction between the calcium hydride and water and then, we will find the weight of calcium hydride and from that molecular weight, we will see how much hydrogen gas is released and then, by applying unitary method, we can find the weight of hydrogen gas obtained from $42g$ of $Ca{{H}_{2}}$. Now solve it.

Complete Solution :
First of all, we will write the reaction between the calcium hydride and water.
When calcium hydride is made to undergo reaction with the water, it forms the combination reaction and results in the formation of calcium hydroxide along with the formation of hydrogen gas.
The reaction is supposed to occur as:
$Ca{{H}_{2}}+2{{H}_{2}}O\to Ca{{(OH)}_{2}}+2{{H}_{2}}$

Now, we will find the molecular weight of calcium hydride i.e. $Ca{{H}_{2}}$
Molecular weight of $Ca{{H}_{2}}$ = $40 + 2 = 42$ gm
And Molecular weight of hydrogen gas = $2$ gm

Now, from the reaction we can see that;
$42$ gm of the calcium hydride gives = $2$ gm of hydrogen gas
Then; $1$ gm of the calcium hydride gives = $\dfrac{2}{42}$ gm of hydrogen gas
And $42$ gm of the calcium hydride gives = $\dfrac{2}{42}\times 42 = 2 gm$ of hydrogen gas.
Hence, the weight of hydrogen gas obtained from $42g$ of $Ca{{H}_{2}}$ by treatment with water comes out to be $42$ gm.

Note: Combination reactions are those reactions in which two reactants combine together to form one main product along with the side product which can be either salt or a gas.
Consider the general reaction as:
$A+B\to C+\text{D(gas)}$