Question

1.) You are given a sample of an unknown hydrate of CaSO4. The average values of the data collected and recoded in your laboratory notebook where as follows: Mass of empty crucible 31.138 g Mass of crucible and sample before heating 31.457 g Mass of crucible and sample after heating 31.436 g Calculate the formula of the CaSO4 hydrate.

2.) Sodium hydroxide pellets are very hygroscopic. Describe what happens chemically when a cover is left off a bottle of NaOH pellets. (Hint: Carbon dioxide reacts with NaOH.)

3.) A 120.0 mg sample of a lead acetate hydrate, Pb(C2H3O2)2  xH2O was heated to drive off the waters of hydration. The cooled residue weighed 103.0 mg. Calculate the value of x in the acetate formula.

Answer 1

1)

Mass of empty crucible = 31.138 g

Mass of crucible and sample before heating = 31.457 g

mass of sample = 31.457 - 31.138

                          = 0.319 g

Mass of crucible and sample after heating = 31.436 g

mass of water after heating = 31.457 - 31.436

                                               = 0.021 g

mass of water = 0.021 g

moles of water = 0.021 / 18

                           = 1.17 x 10^-3

mass of CaSO4 = 0.319 - 0.021

                          = 0.298 g

moles of CaSO4 = mass / molar mass of CaSO4

                          = 0.298 / 136.14

                      = 2.19 x 10^-3

moles of CaSO4 : moles of water

2.19 x 10^-3    :   1.17 x 10^-3

        2        :        1

        1                 :         1/2

formula of the CaSO4 hydrat = CaSO4.1/2H2O

2)

NaOH will eats the bottle cover.

3)

Pb(C2H3O2)2 xH2O

Pb(C2H3O2)2 2H2O

x = 2