KEY2CHEM

Gravimetric Analysis

In gravimetric analysis, the amount of a substance being analyzed (the analyte) can be determined by measuring mass. The mass of solid formed from the reaction of analyte with another substance can be used to determine the concentration of the analyte in the initial sample.

Example 1.

How many grams of $$\require{mhchem}\ce{PbI2(s)}$$ can be formed when $$15.0\text{ mL}$$ of $$0.27\text{ M}$$ $$\require{mhchem}\ce{Pb(NO3)2(aq)}$$ reacts with excess $$\require{mhchem}\ce{KI(aq)}$$ by the following balanced equation?

$$\require{mhchem}\ce{Pb(NO3)2(aq) + 2KI(aq) -> PbI2(s) + 2NaNO3(aq)}$$

A. $$1.87\text{ g}$$

B. $$1870\text{ g}$$

C. $$3.46\text{ g}$$

Solution

A. $$1.87\text{ g}$$

The mass of solid product can be determined from the given quantity of starting material.

$$15.0\text{ mL }\require{mhchem}\ce{Pb(NO3)2} \times \frac{1\text{ L}}{1000\text{ mL}} \times \frac{0.27\text{ mol }\ce{Pb(NO3)2}}{1\text{ L }}\times \frac{1\text{ mol }\ce{PbI2}}{1\text{ mol }\ce{Pb(NO3)2}}\times \frac{461.02\text{ g }\ce{PbI2}}{1\text{ mol }\ce{PbI2} }= 1.87\text{ g}$$

Example 2.

What volume of $$0.18\text{ M}$$ $$\require{mhchem}\ce{CaCl2(aq)}$$ is needed to react with excess $$\require{mhchem}\ce{Na2CO3(aq)}$$ to generate $$6.21\text{ g}$$ $$\require{mhchem}\ce{CaCO3(s)}$$

$$\require{mhchem}\ce{CaCl2(aq) + Na2CO3(aq) -> CaCO3(s) + 2NaCl(aq)}$$

A. $$345 \text{ mL}$$

B. $$112 \text{ mL}$$

C. $$271 \text{ mL}$$

Solution

A. $$345 \text{ mL}$$

Molar mass converts between mass and moles. The subscripts in the formula indicate the relative ratio of atoms. Avogadro's number converts between moles and atoms.

$$6.21\text{ g }\require{mhchem}\ce{CaCO3} \times \frac{1\text{ mol }\ce{CaCO3}}{100.09\text{ g }\ce{CaCO3}}\times \frac{1\text{ mol }\ce{CaCl2}}{1\text{ mol }\ce{CaCO3}}\times \frac{1\text{ L}}{0.18\text{ mol }} \times \frac{1000\text{ mL}}{1\text{ L}}=345 \text{ mL}$$

Example 3.

A solid product can be isolated from an aqueous reaction mixture by ______.

A. sublimation

B. filtration

C. distillation

Solution

B. filtration

A solid can be effectively separated from an aqueous reaction mixture by filtration. Gravity filtration or suction filtration can be used, depending on the available equipment.