Question

If a sample of 0.140 g of KCN is treated with an excess of HCl, how do you calculate the amount of HCN formed in grams?

Answer 1

Hint: Write the balanced chemical equation and find out the mole ratio, then use the formula for calculating the number of moles and find out the weight by substituting data from the question.

Complete answer:
In order to answer the question, we need to know about moles and molar mass. Now, matter is made up of atoms, and as matter has mass, then the atoms should have an individual mass. Molar mass of an element or compound is the mass which houses $6\times {{10}^{23}}$ particles. For, example, the hydrogen molecule has a molar mass of 2 grams. This means 2 grams of hydrogen contains $6\times {{10}^{23}}$atoms, and this number is also called the Avogadro’s number.
Number of moles of an element or a compound is the ratio of its given mass taken by the user, to its molar mass. More is the number of moles, more is the concentration of the substance. Now, let us come to the question. Let us write the chemical equation, for the reaction and then analyse it:
\[KCN(aq)+HCl(aq)\to KCl(aq)+HCN(g)\]
On comparing the mole coefficients, we can observe that 1 mole of KCN produces 1 mole of HCN gas. The molar mass of potassium cyanide is $65g\,mo{{l}^{-1}}$, and the number of moles is $\dfrac{0.140}{65}=2.15\times {{10}^{-3}}mol$. As there is a $1:1$ equivalence, so, in order to find the amount or mass of HCN formed, we multiply this number of moles with the molecular mass of hydrogen cyanide. We can find the number of moles by the formula:
\[no\,of\,moles=\dfrac{given\,mass}{molar\,mass}\]
So, by using the formula, the mass of hydrogen cyanide that is formed is $2.15\times {{10}^{-3}}mol\times 27.03g=58.1mg$. This means that when a sample of 0.140 g of KCN is treated with an excess of HCl, 58.1 mg of HCN is formed.

Note:
The hydrogen cyanide gas is a very poisonous gas, and therefore it is used in the application of rat poison. It has an odour similar to almonds.