Question

Consider this reaction involving an unknown element X.

F 2 + 2 XBr ⟶ Br 2 + 2 XF

When 7.000 g of XBr reacts, 4.700 g of Br 2 is produced. Calculate the molar mass of X, and then identify its element symbol.

Answer 1

2 mole of XBr gives 1 mole of Br2.

Now molecular weight of Br = 80g/mol

So Molecular weight of Br2 = 160 g/mol

Moles of Br2 produced = 4.7/160

Moles of XBr required will be 2*4.7/160 = 0.05875

So 7/(x+80) = 0.5875

X = 39 g/mol

So the element is potassium ( K)