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

Md Ariful Islam

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The molar mass of $Mg(OH)_2$ is $58.3$ $gmol^-1$. The formula mass contains one mole of $Mg$ $(24.3$ $gmol^-1)$ and two moles each of $O$ $(16$ $gmol^-1)$ and hydrogen $H$ $(1$ $gmol^-1)$.

That means the percentages of each element are:

%Mg = $24.3/58.3xx100 = 41.7$ %
%O = $(2xx16)/58.3xx100=54.9$ %
%H = $(2xx1)/58.3xx100=3.4$ %

These percentages add to 100%.

The mass of magnesium in $97.4$ $g$ of $Mg(OH)_2$ is $41.7$ % pf $97.4 = 40.6$ $g$.

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