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 .

Solution :

It is given that :

The formula of the compound =

And that 4.70 g of the sample contains mol of Br.

It means that :

1 mol of contains = 3 mol of Br

∴ 3 mol of contain in 1 mol of

mol of  contains mol of

Thus the mol of = mol

The given mass is = 4.700 g

Therefore, the of  

                                                          = 298.4 g/mol

So of = of X + 3 x of Br

      298.4 g/mol = of X + 3 x 79.90 g/mol

      298.4 g/mol = of X + 239.7 g/mol

        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.

"standard molar enthalpy:

h2o ( liquid water ) : - 285.8 kj/mole

h2o ( water vapor ) : - 241.8 kj / mole.

δhrxn = - 241. 8 - ( - 285.8 )= -241.8 + 285.8 = 44.0

answer: 44.0 kj"