f.4 chemistry

[匿名]

2013-02-18 7:33 pm

when 100g of pure calcium carbonate(formula mass =100.1) reacted with excess hydrochloric acid, 44g of carbon dioxide were obtained.
However, in a similar experiment using 100g of impure calcium carbonate, 46g of carbon dioxide were obtained, what would this impurity be?

A. MgCO3 (*84.3)
B. ZnCO3 (*125.4)
C. FeCO3 (*115.8)
D. BaCO3 (*197.3)

*= formular mass

回答 (1)

賣女孩的火柴

2013-02-19 5:47 am

✔ 最佳答案

The answer is : A. MgCO3

All the metal carbonates used have the formula XCO3, and thus theequation for the reactions between the carbonate and the acid is :
X2CO3 + 2HCl → 2XCl + H2O + CO2

The mole ratio of X2CO3 : CO2 = 1 : 1
This means that the more the number of moles of X2CO3,the greater the mass of CO2 produced.

No. of moles = (mass) / (molar mass)
If a metal carbonate with formulamass less that of CaCO3, the number of moles of this metalcarbonate will be more than that of CaCO3, and thus the mass of CO2produced (46 g) will be greater than that (44g) when CaCO3 is used.

Among the metal carbonates in the four options, only MgCO3 (option A)has formula mass (84.3) less than that of CaCO3 (100.1). Hence, the key is (A).

參考: 賣女孩的火柴

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