It is not important to accurately measure the amount of HCl placed in the eudiometer tube since HCl will be added in excess to react with all the moles of Mg.
In gas stoichiometry, through the method of displacement of a liquid (generally water), the gaseous byproduct is collected inside a long, thin graded glass tube called a eudiometer.
If we consider a reaction between Magnesium and Hydrochloric acid to give a product known as magnesium chloride and hydrogen gas, we can have the chemical equation represented as:
From the above reaction, for each mole of Magnesium taking place in the reaction, 1 mole of hydrogen gas is also produced.
Thus, we can have a prediction that HCl can always be added in excess in order for us to react to all the moles of solid magnesium, hence, it is not important to measure the moles of HCl since it will be added in excess.
Learn more about gas stoichiometry here:
