Gravimetric Analysis Problems, 4

Category: Chemical Engineering Math

"Published in Newark, California, USA"

What weight of an impure NaCl sample must be taken for analysis so that the weight of AgCl precipitate obtained in mg will be equal to the % Cl in the sample?

Solution:

The description of a given problem is about gravimetric analysis in which the amount of an impure NaCl is unknown. If the amount of impure NaCl sample is dissolved in water and then add with AgNO₃ solution, then AgCl precipitate will be formed as follows

By using gravimetric analysis, if x is the weight of AgCl crystals in mg, then the weight of Cl in AgCl is

If the weight of AgCl precipitate is equal to the % Cl in the sample, therefore the weight of impure NaCl sample is
 

 

 

 

 

Gravimetric Analysis Problems, 3

Category: Chemical Engineering Math

"Published in Newark, California, USA"

The aluminum in a 759.08 mg of impure ammonium aluminum sulfate sample was precipitated as Al(OH)₃ and ignited at 1100°C to yield a precipitate of Al₂O₃ weighing 387.953 mg. Express the result of analysis in terms of % Al.

Solution:

From the description of a given problem, it is about gravimetric analysis because it involves the analysis of a certain element or a compound in a sample. Most of the sample is either impure or a mixture of two or more compounds. In this case, we will use the molecular weights as their factors to solve for the weight of a pure substance or a compound. 

The weight of Al in Al₂O₃, we have 

Therefore, the % Al in a sample is

Gravimetric Analysis Problems, 2

Category: Chemical Engineering Math

"Published in Newark, California, USA"

A sample containing NaBr and KBr only weighs 253.02 mg. The sample was dissolved in water and treated with excess AgNO₃. The precipitate formed was found to weigh 429.85 mg. Calculate the %NaBr in the sample.

Solution:

If the solution of the mixture of NaBr and KBr is treated with AgNO₃, then AgBr yellow crystal will be formed which is insoluble in water. The chemical reaction for the formation of AgBr is written as follows

If the word statement says "A sample containing NaBr and KBr only weighs 253.02 mg", then the first working equation is

where x is the weight of NaBr and y is the weight of KBr in the mixture.

By gravimetric analysis, we can calculate the amount of AgBr crystals formed from the mixture of NaBr and KBr using their molecular weights as their factor. 

The amount of AgBr formed from NaBr is

The amount of AgBr formed from KBr is

If the word statement says "The precipitate formed was found to weigh 429.85 mg.", then the next working equation is

Let's consider the two working equations as follows

Multiply the first equation by -1.577873 and then add it to the second equation, we have

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Hence, the weight of NaBr in the mixture is 0.123948 grams.

Therefore, the %NaBr in the mixture is