Ch 205 Lab Help

Titration of Citric Acid in Fruits

We start out by standardizing NaOH with KHP (KHC8H4O4); the pertinent reaction is:

KHC8H4O4(aq) + NaOH(aq) ---> KNaC8H4O4(aq) + H2O(l)

We have the NaOH in the buret and the KHP in the beaker; since we know the mass of each KHP sample titrated, we know the moles of KHP. At the equivalence point in the titration, moles KHP = moles NaOH, and to calculate the molarity of the NaOH, we take mol NaOH / volume NaOH delivered in liters.

You should have three titrations which agree to within 0.001M. In your report, prepare a table which contains the mass of each KHP sample, the volume of NaOH required to titrate the sample, and the calculated molarity of the NaOH. Also report the average molarity of the NaOH; you will need this quantity for part II, Citric Acid Content of Citrus Juices.

For part II, you will use the standardized NaOH to titrate citric acid in a juice sample to an endpoint with phenolphthalein. The pertinent reaction is

H3C6H5O7(aq) + 3NaOH(aq) ----> Na3C6H5O7(aq) + 3H2O(l)

Citric acid is a triprotic acid, and we will be titrating all three protons. We know the average molarity of the NaOH from part I; we also know the volume of NaOH required to reach the endpoint in each titration. Since molarity * volume (in liters!) = mol NaOH, we know how many moles of NaOH were delivered to neutralize the acid. SInce the acid is triprotic, we convert mol NaOH --> mol citric acid using the balanced reaction above:

mol NaOH at equivalence point * ( 1 mol citric acid / 3 mol NaOH) = mol citric acid

To calculate the molarity of the citric acid, divide mol citric acid by the volume of the acid titrated. You report should include a table with the volume of the acid sample(s) and the volume of NaOH required to reach the endpoint, as well as the calculated molarity of the acid.

Next, we must calculate the weight percentage of acid in the sample. We know the number of mol citric acid in the sample (we computed it above); convert this quantity to grams using the molar mass of citric acid. We measured the density of a 5.00 mL sample of juice by weighing it; divide the mass of 5.00 mL of juice (in g) by the volume (in mL) to get the density. Then use the density to find the mass of the juice sample which you titrated (you may have titrated a 4.00 or 10.00 mL sample), and the weight % citric acid is given by

wt % = g citric acid in sample / weight of sample

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