Question

chem pre-lab on stoichiometry and Limiting Reactants

3) During the lab this week, a constant amount of Mg metal (~0.080 g) will be reacted with the two acids HCI or H,SO4 to produce hydrogen gas. a. Write out a correctly formatted hypothesis that describes how the amount of hydrogen gas produced with be affected by the concentration of acid. b. Write out a correctly formatted hypothesis that describes how the amount of hydrogen gas produced with be affected by the type of acid, HCI or H,SO reacted with the Mg metal. 4

Answer 1

The reaction of Magnesium metal with HCl or H₂SO_​​​4 is

Mg (s) + 2 HCl(aq) ---------> MgCl_​​​2 (aq) + H_{​​​​​​2} (g)

Mg (s) + H₂SO₄ (aq) ---------> MgSO₄ (aq) + H_{​​​​​​2} (g)

Reaction with HCl:

24 g of Mg liberates 22.4 L of Hydrogen gas when treated with excess of acid.

0.08 grams of Mg liberates how many litres of hydrogen gas

= 0.08 g * 22.4 L/24 g

= 0.075 L = 0.0067 grams

Effect of concentration:

According to balanced equations,

24 grams of Mg on reaction with 2 moles of HCl liberates 22.4 L of Hydrogen gas.

24 grams of Mg on reaction with 1 moles of Sulphuric acid liberates 22.4 L of Hydrogen gas.

For a given mass of magnesium, if concentration of acid is decreased, number of moles of acid decreases. Hence number of moles of hydrogen gas liberated decreases.

Similarly if concentration of acid is increased, number of moles of acid increases. Hence number of moles of hydrogen gas liberated increases.

Effect of nature of acid:

We know that HCl is a mono protic acid where as sulphuric acid is a diprotic acid.

Suppose let us assume concentration of both the acids are

1 M.

Number of moles of Hydronium ions in 1 M HCl is 1 mole

Number of moles of Hydronium ions in 1M H₂SO_​​​4 is 2 moles

If 1 mole of HCl is reacted with enough quantity of Magnesium, 0.5 moles of Hydrogen gas is liberated.

If 1 mole of Sulphuric acid is reacted with enough quantity of Magnesium, 2 moles of Hydrogen gas is liberated.

How much quantity of Hydrogen gas liberated varies from acid to acid. The different acids which have same basicity, liberates same quantity of Hydrogen gas. Ex: HBr, HCl liberates same number of moles of Hydrogen gas, even though they are different as their basicity is same.