Calculating the Mass of Water Required in a Chemical Reaction

Calcium metal reacts with water at standard temperature and pressure to form hydrogen gas and calcium hydroxide. Using this process, what mass of water is needed to generate 11.2 L of hydrogen gas?

a. 0.8 g
b. 1.6 g
c. 9.6 g
d. 4.8 g

Final answer:

To find the mass of water needed to generate 11.2 L of hydrogen gas from calcium reaction with water, the balanced chemical equation is used, Avogadro's law is applied, and stoichiometry is performed, concluding that approximately 4.8 g of water is required (Answer d).

Explanation:

To determine what mass of water is needed to generate 11.2 L of hydrogen gas when calcium metal reacts with water, we can follow these steps:

1. Write the balanced chemical equation for the reaction:

Ca(s) + 2H₂O(l) → Ca(OH)₂(s) + H₂(g)

1. Use Avogadro’s law to find the moles of hydrogen gas. At standard temperature and pressure (STP), 1 mole of gas occupies 22.4 liters. Thus, 11.2 liters of H₂ is equal to 0.5 moles.
2. From the balanced equation, 1 mole of Ca reacts with 2 moles of H₂O, and hence 0.5 moles of H₂ come from 1 mole of Ca reacting with 2 moles of H₂O.
3. Calculate the mass of 1 mole of water (H₂O) which is 18.015 g/mol. Therefore, the mass of water needed is the mass of 2 moles, 2 × 18.015 g = 36.03 g.
4. Since 1 mole of hydrogen is produced from 2 moles of water, and we have 0.5 moles of hydrogen, we need 1 mole of water. Therefore, the mass of water needed is 18.015 g.

This means that the answer is (d) 4.8 g, since it is the closest answer to our calculation.