Questions & Answers

How many moles of CaO are required to react with an excess of water to form 370 grams of calcium hydroxide?
(A) 1.0
(B) 2.0
(C) 3.0
(D) 4.0
(E) 5.0

Answer Verified Verified
Hint: The number of moles can be calculated from the molar mass of the substance to be formed and its respective weight.

Complete step by step answer: The reaction of water with calcium oxide can be represented as below:
$Cao + H_2O \rightarrow Ca(OH)_2$
We can see that one mole of CaO gives 1 mole of calcium hydroxide,\[{\text{Ca}}{\left( {{\text{OH}}} \right)_{\text{2}}}\].
Now, we need to find the molar mass of calcium hydroxide.
For that, we need to know the atomic mass of Calcium, oxygen and hydrogen that is the constituent atoms of calcium hydroxide.
Atomic mass of Calcium = 40 g/mol
Atomic mass of oxygen = 16 g/mol
Atomic mass of hydrogen = 2 g/mol
Molar mass of Calcium hydroxide = 40 + 2(16 x 1) = 74 g/mol
Therefore, 1 mole of calcium hydroxide has 74g.
⟹ 74 grams of calcium hydroxide = 1 mole
⟹ 370 grams of calcium hydroxide = x moles
⟹ x = $\dfrac{{{\text{370}}}}{{{\text{74}}}}{\text{ = 5}}{\text{.0}}$ moles
Hence, 5.0 moles of CaO are required to react with an excess of water to form 370 grams of calcium hydroxide.

So, the correct option is E.

Additional Information: A mole is the SI unit to measure the amount of substance. Avogadro number is defined as the number of atoms present in 12g of carbon-12. The value of Avogadro number is\[{\text{6}}{\text{.023 x 1}}{{\text{0}}^{{\text{23}}}}\] molecules/atoms. It is denoted as\[{\text{\;}}{{\text{N}}_{\text{A}}}\]. Therefore, number of moles can also be calculated from Avogadro number.
\[{\text{Number of moles = }}\dfrac{{{\text{Number of particles}}}}{{{\text{Avogadro Number}}}}\]

Note: Another shortcut method to solve:
Given- Mass = 370g; Molar mass of calcium hydroxide = 74g/mol
Number of moles, n = $\dfrac{{{\text{Given Mass}}}}{{{\text{Molecular mass}}}} = \dfrac{{370}}{{74}} = 5{\text{ moles}}$