Free counters!

Friday, May 29, 2015

Volumetric Analysis Problems, 3

Category: Chemical Engineering Math

"Published in Newark, California, USA"

A sample consisting of Na2CO3, NaHCO3 and inert matter weighs 1.179 grams. It is titrated with 0.1 N HCl with phenolphthalein as the indicator, and the solution became colorless after the addition of 24.00 mL. Another duplicate sample was titrated with HCl using methyl orange as indicator. It required 50.25 mL of the acid for the color change from yellow to red. What are the percentage of Na2CO3 and NaHCO3 in the sample?

Solution:

Since a given sample consists of Na2CO3 and NaHCO3, then we have to use two indicators like phenolphthalein and methyl orange. The change of color of an indicator depends with the pH of the end point. The pH of the end point of phenolphthalein is usually 7. It is changed from pink to colorless. The pH of the end point of methyl orange is less than 4 or 5. It is changed from yellow to red color. 

In the given problem, the titration method is done twice in a separate method using equal duplicate samples. This method is applicable if the mixture consists of Na2CO3 and NaHCO3

In the first titration using phenolphthalein as the indicator, then we can solve for the amount of NaHCO3 in a sample. The end point must be a colorless solution. The chemical reaction for the first titration is
 

Hence, the weight of NaHCO3 in the first titration is 
 
 
 

In the second titration using methyl orange as the indicator, then we can solve for the amount of Na2CO3 in a sample. The end point must be a red color solution. The chemical reaction for the second titration is



Hence, the weight of NaHCO3 in the second titration is 




Next, let's check if NaHCO3 is present in the sample as follows


 

Since the right side of the equation is greater than the left side of the equation, then the given sample has NaHCO3. If both sides of the equation are equal, then only Na2CO3 is present in the sample

Hence, the weight of NaHCO3 in the sample is 
 
 

Therefore, the percent of NaHCO3 in the sample is 
 
 

The weight of Na2CO3 in the sample using the chemical equations above is  




Therefore, the percent of Na2CO3 in the sample is