Question

How many moles are in 25 grams of water?

Answer 1

Hint: For the calculation of number of moles present in given grams of any substance, we have to know about the relation between no. of moles and grams which is described as \[{\text{n = }}\frac{{\text{W}}}{{\text{M}}}\].

Complete step by step solution:
For the calculation of number of moles in grams we have to use the equation which shows relation between moles (${\text{mol}}$) and grams (${\text{g}}$) which is described as:
\[{\text{n = }}\frac{{\text{W}}}{{\text{M}}}\] …………..(i)
Where, ${\text{n}}$ = moles of species (in ${\text{mol}}$)
${\text{W}}$= Given weight of substance in grams (${\text{g}}$)
${\text{M}}$ = Molecular or molar mass of the given substance in grams per mole (${\text{g/mol}}$)
Whenever we have to calculate number of moles present in grams of any quantity, we have to divide the given weight (in ${\text{g}}$) of the substance by the molecular or molar mass (${\text{gmo}}{{\text{l}}^{{\text{ - 1}}}}$) of that given substance.
As in the given question we have to calculate how many moles are present in the 25 grams of water and for this we have to divide the given weight of water by the molecular weight / mass of water.
Molecular mass of water (${{\text{H}}_{\text{2}}}{\text{O}}$) = ${\text{1 + 1 + 16 = 18gmo}}{{\text{l}}^{{\text{ - 1}}}}$
Now we will use equation (i) to calculate number of moles, so we get
${\text{n = }}\frac{{{\text{25g}}}}{{{\text{18gmo}}{{\text{l}}^{{\text{ - 1}}}}}}{\text{ = 1}}{\text{.38mol}}$
Hence, $1.38$ moles (${\text{mol}}$) are present in 25 grams of water.

Note: Here some of you may think that how the unit converts into a mole by dividing given weight by molecular weight, so the reason is that grams from the numerical & denominator get canceled and only moles will get left.