1.4: Composition of Mixtures

Learning Target/Content Objectives:

I can explain the quantitative relationship between the elemental composition by mass and the composition of substances in a mixture.
I can use gravimetric precipitation to determine the percent composition of substances or ionic compounds in a mixture

What are Gravimetric Methods?

A group of analytical methods in which the amount of an analyte (another word for a sample) is determined through the measurement of the mass of a pure substance containing the analyte.

What is a precipitate? How does it form?

A precipitate is a solid substance that forms when two aqueous solutions are mixed together. A precipitation reaction is a type of double replacement reaction that forms a precipitate.

For example, AgCl is a precipitate that forms when NaCl solution is added to AgNO3 according to the following precipitation reaction:

NaCl (aq) + AgNO3 (aq) ---------------> NaNO3 (aq) + AgCl (s)

Another example of a precipitation reaction is a reaction between AgNO3 and KCl aqueous solution that produces a white AgCl precipitate see the reaction

What is Precipitation Gravimetry?

Precipitation Gravimetry is a method used to analyze a mixture of ionic compounds for quantitative purposes to determine the percent of an ionic compound or ions in a mixture. It is a method that is used to separate ions in a solution using a precipitation reaction.

Steps in Precipitation Gravimetry:

Preparing the solution
Precipitation
Filtration of the precipitate
Washing the precipitate
Weighing the precipitate
Calculation

Gravimetric Precipitation

In precipitation gravimetry the sample to be analyzed is converted to a precipitate, the precipitate is then filtered and weighed for further calculation purposes. See the diagram below.

Gravimetric Analysis lab to determine the % composition of a compound by mass

Watch the youtube video on hoe to conduct a precipitation gravimetry lab to understand how precipitation technique is used to determine the composition of an ionic compound in a mixture

Practice Question

In the analysis of 4.7011 g of a sample containing NaCL and AgNO3, 0.9805 g of AgCl were precipitated. What is the percentage by mass of the Sodium Chloride in the sample?

Fist you would find the mass of Cl in AgCL as follows: mass of Cl= 0.9805 g AgCl x 35.45 g Cl/ 143.32 g AgCl= 0.2425 g Cl

next, you convert the grams Cl to moles Cl as follows: 0.2425 g Cl x 1 mole Cl/ 35.45 g Cl= 0.006841 moles Cl

then, you use stoichiometric ratios to convert moles Cl to moles NaCl and that is 1 to 1 ratio

According to stoichiometric ratios: moles NaCl= moles Cl = 0.006841 moles

after, convert moles NaCl to grams NaCl as follows: 0.006841 moles NaCl x 58.44 g NaCl / 1 mole NaCl = 3.998 g NaCl

% of NaCl in the sample= mass of NaCl / Total mass of the sample x 100= (3.998 g NaCl / 4.7011 g sample) x 100= 85.04 %

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