Question

How many grams are in 3 mole of ${\text{KBr}}$?

Answer 1

Hint: For the calculation of number of grams present in given moles 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 grams in moles we have to use the equation which shows relation between moles (${\text{mol}}$) and grams (${\text{g}}$) which is described as:
                    \[{\text{n = }}\frac{{\text{W}}}{{\text{M}}}\]
And after rearranging the above equation we get,
                                                      ${\text{W = n \times M}}$ ……….. (i)
Where, ${\text{n}}$ = No. of moles (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 the number of grams present in moles of any quantity, we have to multiply the molecular weight (in ${\text{g/mol}}$) of the substance to the given moles (${\text{mol}}$) of that substance.
As in the given question we have to calculate how many grams are present in the 3 mole of ${\text{KBr}}$ and for this we have to multiply the molecular weight of ${\text{KBr}}$to the number of moles of ${\text{KBr}}$.
Molecular mass of ${\text{KBr}}$ = ${\text{39 + 80 = 119gmo}}{{\text{l}}^{{\text{ - 1}}}}$
Now we will use equation (i) to calculate the grams, so we get
${W = 3mol \times 119}\frac{{\text{g}}}{{{\text{mol}}}}{\text{ = 357g}}$
Hence, $357$ grams (${\text{g}}$) are present in 3 mole of ${\text{KBr}}$.

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