Question

Calcium hydride reacts with water to form calcium hydroxide and hydrogen gas. The balanced equation to this is `CaH_2+2H_2O`----`Ca(OH)_2+2H_2` How many grams of calcium hydride are needed to form 8.400 g of hydrogen?

Answer 1

You need `"87.69 g of CaH"_2` to form `"8.400 g of H"_2`.

Explanation:

There are four steps to answering this type of stoichiometry problem.

1. Write the balanced equation for the reaction.
2. Use the molar mass of `"H"_2` to convert grams of `"H"_2` to moles of `"H"_2`.
3. Use the molar ratio from the balanced equation to convert moles of `"H"_2` to moles of `"CaH"_2`.
4. Use the molar mass of `"CaH"_2` to convert moles of `"CaH"_2` to grams of `"CaH"_2`.

Step 1. Write the balanced equation.

`"CaH"_2 + "2H"_2"O" → "Ca(OH)"_2 + "2H"_2`

Step 2. Convert grams of `"H"_2` to moles of `"H"_2`.

The molar mass of `"H"_2` is 2.016 g/mol.

`8.400 cancel("g H"_2) × ("1 mol H"2)/(2.016 cancel("g H"2)) = "4.1667 mol H"_2`

Step 3. Use the molar ratio to calculate the moles of `"CaH"_2`.

From the balanced equation, the molar ratio is `"1 mol CaH"_2:"2 mol H"_2`.

`4.1667 cancel("mol H"_2) × ("1 mol CaH"_2)/(2 cancel("mol H"_2)) = "2.0833 mol CaH"_2`

Step 4. Convert moles of `"CaH"_2` to grams of `"CaH"_2`.

The molar mass of `"CaH"_2` is 42.09 g/mol.

`2.0833 cancel("mol CaH"_2) × ("42.09 g CaH"_2)/(1 cancel("mol CaH"_2)) = "87.69 g CaH"2`

The reaction requires `"87.69 g CaH"_2`.

This video link allows you to furthur practice similar problems.