Category: Chemical Engineering Math
"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
Category: Chemical Engineering Math
"Published in Newark, California, USA"
In standardizing a solution of NaOH against 1.431 grams of potassium hydrogen phthalate, C8H5KO4 (KHP), the analyst uses 35.50 mL of the alkali and has to run back with 8.25 mL of acid (1 mL = 10.75 mg NaOH). What is the molarity of the NaOH solution?
Solution:
The given word problem is about volumetric analysis problem because it involve the titration of samples and solutions. Potassium hydrogen phthalate (KHP) is a sample used to standardize the basic or alkali solutions that are prepared in the lab.
If NaOH solution is used to titrate KHP sample, then the chemical reaction for neutralization is
The concentration of NaOH solution used to titrate a sample of KHP is
Since, there's a back titration with an acid solution especially if there's an over titration with an alkali solution, then the above equation becomes
Usually, we put the indicator at the sample solution so that we know that we used the right amount of acid or alkali solution by its color. The color of the indicator is changed once it reached its end point. How about if you used the excess solution for titration, then we have to do the back titration until we get the correct color for its end point.
Therefore, the concentration of NaOH solution used to titrate a sample of KHP is
Category: Chemical Engineering Math
"Published in Newark, California, USA"
Calculate the molarity of NaOH solution if 12.25 mL was used to titrate 0.2615 gram of primary standard KHP (potassium hydrogen phthalate, C8H5KO4).
Solution:
The given problem is about finding the concentration of NaOH solution. KHP is an acid mostly used for standardization of any basic solution that are usually prepared in the lab. Although we know how to prepare any solutions in the lab, it is better to do the standardization so that we can check if the concentration is correct or not.
If NaOH solution is used to titrate KHP sample, then the chemical reaction for neutralization is
Therefore, the concentration of NaOH solution used to titrate a sample of KHP is
