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multiple choice HCL + carbonate/nitrate question

31 A mixture of solids is treated with an excess of dilute hydrochloric acid. A colourless gas is evolved and a white precipitate forms. What are the solids in the mixture?
 
A calcium carbonate and magnesium hydroxide
B calcium carbonate and magnesium nitrate
C lead nitrate and calcium hydroxide
D lead nitrate and magnesium carbonate
 
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1 Answer

A carbonate salt must be present to account for the appearance of a colorless gas (CO2).  So, that eliminates (C) as a possibility.  In order to account for the presence of a white precipitate, one looks for an insoluble chloride salt (from the HCl), or some other insoluble material.  Looking at the unbalanced equations, one sees the following:
(A)  CaCO3 + Mg(OH)2 + HCl ==> CaCl2 + MgCl2 + H2O + CO2 + H2O --> a gas is formed but no white ppt
(B)  CaCO3 + Mg(NO3)2 + HCl ==> CaCl2 + MgCl2 + HNO3 + CO2 + H2O --> a gas is formed but not white ppt
(D)  Pb(NO3)2 + MgCO3 + HCl ==> MgCl2 + PbCl2 + HNO3 + CO2 + H2O --> a gas is formed and PbCl2 is a white ppt.
Note however, that in the presence of excess dilute HCl, it is probable that PbCl2 will dissolve as it is slightly soluble in dilute HCl.
 
Probably the best choice for the answer would be (D)