Which of these formulas is least soluble in water?

STAAR Resource Material

A. PbCl2
Correct!

B.  MgCl2
Incorrect. Compounds containing Cl- are almost always soluble, unless they are combined with Ag+, Pb+2, or Hg2+2.

C. Na2CO3
Incorrect. Compounds with CO3-2 are usually insoluble, unless they are combined with an alkali metal or NH4+; Na is an alkali metal.

D. Pb(NO3)2
Incorrect. Compounds containing NO3- are always soluble.

Which ion forms a compound that is insoluble in water when it is combined with sulfate ion, SO4-2?

STAAR Resource Material

A. Na+
Incorrect. Alkali metals always form soluble compounds with SO4-2.

B. Ba+2
Correct!

C. Mg+2
Incorrect. Mg+2 is not listed as one of the exceptions.

D. K+
Incorrect. Alkali metals always form soluble compounds with SO4-2.

Which of the following reactions is an example of a precipitation reaction?

STAAR Resource Material

A. NaOH + HCl → NaCl + H2O
Incorrect. This is an example of an acid-base reaction. No insoluble product is formed.

B. 2KCl+ Pb(NO3)2 → 2KNO3 + PbCl2
Correct! Pb(OH)2 is an insoluble product.

C. 2Al + 3CuSO4 → 3Cu + Al2(SO4)3
Incorrect. This is an example of a redox reaction. No insoluble product is formed.

D. 2Mg + 02 → 2MgO
Incorrect. This is an example of a redox reaction.

Which of the following reactants will produce a precipitate?

STAAR Resource Material

A. LiNO3 +Na2SO4
Incorrect. The products are Li2SO4 and NaNO3, and they are both soluble.

B. NH4Cl + K3PO4
Incorrect. The products are (NH4)3PO4, and KCl and both are soluble.

C. BaS + Ca(ClO3)2
Incorrect. The products are Ba(ClO3)2 and CaS and both are soluble.

D. Na2CO3 + MgCl2
Correct!