Question

gA scientist is trying to discover information about an unknown metal in a compound. The formula for the compound is believed to be XBr3XBr3 where XX is the unknown metal. The scientist determined that a 4.700 g4.700 g sample of this compound contains 4.834×10−2 mol Br4.834×10−2 mol Br . Calculate the atomic mass of the unknown metal, XX .

Answer 1

Solution :

It is given that :

The formula of the compound = $XBr_3$

And that 4.70 g of the sample contains $4.834 \times 10^{-2}$ mol of Br.

It means that :

1 mol of $XBr_3$ contains = 3 mol of Br

∴ 3 mol of $Br^-$ contain in 1 mol of $XBr_3$

$4.834 \times 10^{-2}$ mol of $Br^-$  contains $1.576 \times 10^{-2}$ mol of $XBr_3$

Thus the mol of $XBr_3$ = $1.576 \times 10^{-2}$ mol

The given mass is = 4.700 g

Therefore, the $\text{molar mass}$ of $XBr_3$  $=\frac{\text{mass}}{\text{mol}}$

                                                           $=\frac{4.700 \ g}{1.576 \times 10^{-2} \ mol}$

                                                          = 298.4 g/mol

So $\text{molar mass}$ of $XBr_3$ = $\text{molar mass}$ of X + 3 x $\text{molar mass}$ of Br

      298.4 g/mol = $\text{molar mass}$ of X + 3 x 79.90 g/mol

      298.4 g/mol = $\text{molar mass}$ of X + 239.7 g/mol

       $\text{molar mass}$ of X = 58.71 g/mol    (since 1 amu = 1 g/mol)      

Therefore the atomic mass of the unknown metal = 58.71 g/mol

So the unknown meta is Nickel.