# How many moles are in 2.9 g of magnesium hydroxide?

$\text{Moles"="Mass"/"Molar mass} \cong 0.05 \cdot m o l \cdot {L}^{-} 1$.
$\text{Moles of}$ "Mg(OH)"_2=("2.9"*"g")/("58.32"*g*mol^-1)=0.0497*mol.