Question

What weight of zinc would be required to produce enough hydrogen to reduce $ 8.5g $ of copper oxide completely into copper?

Answer 1

Hint: One of the important aspects of a chemical equation is that when it is written in the balanced form, it gives quantitative relationships between the various reactants and products in terms of moles, masses, molecules and volumes.

Complete answer:
In the question it is given that zinc on reaction produces hydrogen that will reduce $ 8.5g $ of copper oxide completely into copper. We have to determine the weight of zinc required.
Let’s see the chemical equations of the reactions first;
Zinc $ \left( {Zn} \right) $ reacts with an acid (say $ HCl $ ) to evolve hydrogen gas;
 $ Zn + 2HCl \to ZnC{l_2} + {H_2} $
Copper oxide then reacts with hydrogen gas and gets reduced to copper;
 $ CuO + {H_2} \to Cu + {H_2}O $
We can write the overall chemical equation as;
 $ Zn + 2HCl + CuO \to ZnC{l_2} + Cu + {H_2}O $
Zinc on reaction produces hydrogen gas which then reacts with copper oxide and reduces it to copper. We can say that;
 $ 79.5g $ of $ CuO $ requires $ 65g $ of $ Zn $
 $ 79.5g $ = Molar mass of $ CuO $
 $ 65g $ = Molar mass of $ Zn $
Using this;
 $ 79.5g $ of $ CuO $ $ \to $ $ 65g $ of $ Zn $
 $ 8.5g $ of $ CuO $ $ \to $ $ \dfrac{{65 \times 8.5}}{{79.5}}g $ of $ Zn $
 $ \to 6.95g $
The amount of Zinc required is $ 6.95g $

Additional Information:
The qualitative information conveyed by a chemical equation helps in a number of calculations. The problems involving these calculations may be classified into; mass-volume relationship in which the mass/volume of one of the products is given and the volume/mass of the other needs to be calculated, volume-volume relationship and mass-mass relationship.

Note:
In this question we used the mass-mass relationship in which the mass of one of the reactants/ product is given and the mass of some other reactant or product is to be calculated. And the mass of Zinc required was found to be $ 6.95g $