Question

On adding 5.1 gram of an impure sample of magnesium to an excess of HCl ,3.74 of hydrogen gas evolved
Calculate the mass percentage of magnesium

Answer 1

Mg + 2HCl --> MgCl2 + H2

You formed 3.74 grams of hydrogen gas.
You need to calculate how many moles of H2 you have first.

The molar mass of hydrogen gas is 2.016

That means 2.016 grams of H2 is 1 mole

How many moles is 3.74 grams?
(Just divide it by 2.016 to get your answer.)

That's also going to be how many moles of magnesium you have since the reaction is 1 mole of magnesium produces 1 mole of hydrogen gas

Answer 2

You then multiply by the molar mass of magnesium which is 24.305 to determine how many grams of magnesium reacted to form the hydrogen gas.

Once you have that number, you divide it by 5.1 and multiply by 100 to find the mass percentage of magnesium in the impure sample.

Essentially
\(\sf mass ~\text{%} = \dfrac{magnesium~mass}{total~mass}\times 100\)

and the total mass is 5.1 g
Once you find the mass of magnesium, you can calculate the mass percent