"Published in Newark, California, USA"
A sample consisting of Na2CO3, NaOH, and inert matter weighs 1.179 grams. It is titrated with 0.2239 M HCl with phenolphthalein as the indicator, and the solution became colorless after the addition of 45.62 mL. Methyl orange is then added and 12.85 mL more of the acid are needed for the color change. What are the percent of Na2CO3 and NaOH in the sample?
Solution:
Since a given sample consists of Na2CO3 and NaOH, 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.
If a sample is added with phenolphthalein and titrated with HCl solution, then the solution changed its color from pink to colorless. The chemical reaction for the first titration is
Sodium hydroxide is neutralized completely and the endpoint is colorless. The weight of NaOH in a sample is
Since Na2CO3 is converted into NaHCO3 and NaHCO3 is not yet a stable salt, then we need to do the titration again by adding methyl orange and then titrate with HCl solution until the color is changed from yellow to red. The chemical reaction for the second titration is
The weight of NaHCO3 is
By gravimetric analysis and from the chemical equations above, the weight of Na2CO3 is
Therefore, the percent of Na2CO3 in a sample is
and the percent of NaOH in a sample is
