Question

What mass of hydrogen atoms are in 166 g of zinc hydroxide $Zn{{(OH)}_{2}}$(Mw. 99.43) ?

Answer 1

Hint: Molecular mass of any compound tells the sum of mass of all the atoms present in that compound. Number of moles can be calculated from the given mass upon molecular mass. Thereby the mass unknown can also be determined by using the molecular mass and number of moles.
Formula used:
Number of moles = $\dfrac{given\,mass}{molar\,mass}=\dfrac{w}{m}$

Complete answer:
We have been given a mass of 166 grams of zinc hydroxide whose formula is $Zn{{(OH)}_{2}}$. We have to find the mass that is occupied by the hydrogen atoms in this given mass. For this we will first take out the number of moles of zinc hydroxide from the number of moles formula with the given mass and molar mass of zinc hydroxide (99.43 g/mol). Then from these numbers of moles, we will take out the mass of hydrogen present in 166 g of zinc hydroxide, by multiplying the number of moles with the molar mass of hydrogen.
So, calculating number of moles as $\dfrac{given\,mass}{molar\,mass}=\dfrac{w}{m}$
Number of moles = $\dfrac{166\,g}{99.43\,g\,mo{{l}^{-1}}}$
Number of moles of $Zn{{(OH)}_{2}}$= 1.67 moles
Now, as we can see there are 2 hydroxide ions that consist of 2 hydrogen atoms, so, multiplying the number of moles by 2 and the molar mass of hydrogen, we will get the mass of hydrogen as
Mass = 1.67 moles$\times $2$\times $1 g/mol
Mass = 3.37 g
Hence, the mass of hydrogen in 166 g of zinc hydroxide $Zn{{(OH)}_{2}}$is 3.37 g.

Note:
The molecular masses of elements and compounds are written in gram per mole as they denote the mass of 1 mole of that substance. 1 mole of any substance consists of$6.022\times {{10}^{23}}$atoms or particles, which are also called Avogadro numbers. In 1 mole of anything, the mass is equal to the molar mass of that substance.