Lead (Pb)
Let's figure out how many moles of the unknown element there is and what it's atomic weight is.
We have 0.3253 mol of mBr4 which has a mass of 171.4 g. Since there's 4 bromine atoms per molecule, that means we have
4 * 0.3253 = 1.3012 moles of bromine.
The atomic weight of bromine is 79.904, so
1.3012 mol * 79.904 g/mol = 103.9711 g
So now subtract the known mass of bromine from the mass of the unknown substance.
171.4g - 103.9711g = 67.42892 g
Now divide that by the number of moles we have of that metal to determine its atomic weight
67.42892 g / 0.3253 mol = 207.2822 g/mol
Now look at the periodic table to see if there's any metals with an atomic weight close to 207.2822 g/mol
The closest match is for Lead (Pb) with an atomic weight of 207.2. Additionally, it's in the same column as Carbon so Lead has 4 valance electrons. And Bromine wants to grab one electron so you'd expect lead to combine with 4 bromine atoms. Which this compound does. So the evidence indicates that the unknown metal is Lead.
