How many moles of H2 can be formed if a 4.71 g sample of Mg reacts with excess HCl?

The number of moles of H₂ that can be formed if a 4.71 g sample of Mg reacts with excess HCl is 0.19625 moles

Let's represent the reaction with a chemical equation

Mg + HCl → MgCl₂ + H₂

Mg + 2HCl → MgCl₂ + H₂

HCl is in excess according to the question. This means Mg is the limiting reagent and therefore determine the amount of product.

Therefore,

24 g of Mg gives 2g of  H₂

4.71 g of Mg will give ? H₂

cross multiply

mass of H₂ = 4.71 × 2 / 24

mass of H₂ = 9.42 / 24 = 0.3925 g

moles of H₂ = mass / molar mass

moles of H₂ = 0.3925 / 2

moles of H₂ = 0.19625 moles

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0.2 mole of H2.

Explanation:

We'll begin by calculating the number of mole in 4.71 g of magnesium (Mg). This can be obtained as follow:

Mass of Mg = 4.71 g

Molar mass of Mg = 24 g/mol

Mole of Mg =?

Mole = mass /Molar mass

Mole of Mg = 4.71 /24

Mole of Mg = 0.2 mole

Next, we shall write the balanced equation for the reaction.

This is illustrated below:

Mg + 2HCl —> MgCl2 + H2

From the balanced equation above,

1 mole of Mg reacted to produce 1 mole of H2.

Finally, we shall determine the number of mole of H2 produced from the reaction. This can be obtained as illustrated below:

From the balanced equation above,

1 mole of Mg reacted to produce 1 mole of H2.

Therefore, 0.2 mole of Mg will also react to produce 0.2 mole of H2.

Thus, 0.2 mole of H2 was obtained from the reaction.

i believe you are right for all except the 3rd ( look at sqdancefan his answer is much better)