CHEM F.4 -------------------------40 marks

No. of moles of CaSO4 = 9.8 / (40 + 32.1 + 16 x 4) = 0.072mol

No. of moles of Ca2+ ions = 0.072 x 1 = 0.072mol

To have the same number of Ca2+ ions, the number of moles of Mg2+ should be
0.072mol

Thus, the required mass should be 0.072 x 24.3 = 1.75g


For Fe3O2, should it be Fe2O3? Fe3O2 should not be exist as there is no O3- ion

For Fe2O3, it has 1 Fe2O3 particle, 5 atoms (2 Fe atoms and 3 O atoms), and also 5 ions (2 Fe3+ ions and 3 O2- ions)

1.
mole of CaSO4 of 9.08g of CaSO4
9.08/40.08 + 96 = 0.067

There are 0.067 mole of Ca2+ ion in 0.67 mole CaSO4

Mass of 0.067 Mg2+:

0.067*24.305 = 1.628g


1.628gof Mg2+ ions contain the same number of Ca2+ ions as 9.08g
of CaSO4

2007-03-31 22:52:03 補充：
there are Fe3O2, it is a mixture oxide i think. But it just be Fe3O2 in the world, people above are wrong.However, i don't konw much more about Fe3O2, sorry!

2007-03-31 22:52:15 補充：
But u can search it on yahoo, i find some data on it.And hope this will help uhttp://www.newton.dep.anl.gov/askasci/chem03/chem03643.htm

no. of mole of CaSO4 = 9.08/(40.1+32.1+16*4) = 1/15 = 0.0667
no. of mole of Ca2+ in CaSO4 = 1/15 = 0.0667
since no. of Mg2+ = no. of Ca2+
no. of mole of Mg2+ = no. of mole of Ca2+ = 1/15 = 0.0667
mass of Mg2+ = 1/15 * 24.3 = 1.62g

Let m be the no. of mole of Fe3O2
no. of ions in Fe3O2 = 2m
no. of atoms = 0 (there is no atoms because all the atoms have been changed to ions)

2007-03-31 21:50:59 補充：
actually i also think that it should be Fe2O3but the no. of ions is still 2m1 for Fe3+, 1 for O2-and the no. of atoms is still 0