Question

Calculate the percentages by mass of magnesium, oxygen and hydrogen in magnesium hydroxide, $ Mg{\left( {OH} \right)_2} $ . What is the mass of magnesium in $ 97.4g $ of $ Mg{\left( {OH} \right)_2} $ ?

Answer 1

Hint: Magnesium hydroxide is a compound consisting of one magnesium, two oxygen and two hydrogen atoms. Based on the molar mass of magnesium hydroxide and each atom molar mass the percentage of each atom can be determined. Based on the percentage and given amount the mass of each atom present in the given mass can be known.

Complete answer:
Magnesium combines with water and forms magnesium hydroxide. Magnesium hydroxide has the chemical formula of $ Mg{\left( {OH} \right)_2} $ . It consists of two atoms of hydrogen, two atoms of oxygen and one atom of magnesium.
The molar mass of magnesium is $ 24.3amu $ , the molar mass of oxygen is $ 16 $ and the molar mass of hydrogen is $ 1 $
The percentage of magnesium in $ Mg{\left( {OH} \right)_2} $ is $ \dfrac{{24.3}}{{58.3}} \times 100 = 41.7% $
The percentage of oxygen in $ Mg{\left( {OH} \right)_2} $ is $ \dfrac{{2 \times 16}}{{58.3}} \times 100 = 54.9% $
The percentage of hydrogen in $ Mg{\left( {OH} \right)_2} $ is $ \dfrac{{2 \times 1}}{{58.3}} \times 100 = 3.4% $
Given that the mass of $ Mg{\left( {OH} \right)_2} $ is $ 97.4g $
The percentage of magnesium in $ Mg{\left( {OH} \right)_2} $ is $ 41.7% $
The amount or mass of magnesium present in $ 97.4g $ of $ Mg{\left( {OH} \right)_2} $ is $ 41.7% $ of $ 97.4g $ which can be written as $ \dfrac{{41.7}}{{100}} \times 97.4 = 40.6g $
Thus, the mass of magnesium in $ 97.4g $ of $ Mg{\left( {OH} \right)_2} $ is $ 40.6g $ .

Note:
The molar mass of each atom must be taken exactly along with the significant figures or digits in decimals. While calculating the percentages of each atom in the given compound the number of atoms should also be considered. Thus, the molar mass of oxygen and hydrogen should be multiplied with two as there were two atoms of oxygen and hydrogen.