Okay so my professor gave us the data collected for a lab in which cola was titr

Question

Okay so my professor gave us the data collected for a lab in which cola was titrated with NaOH and from that we are supposed to determine how much of phosphoric acid is in cola.

Data Given:

Initial temp: 24.8 degrees C

Concentration of NaOH: 0.010M                  Trial 1        Trial 2

Exact Volume of cola:                                 20.0mL      20.0mL

pH of cola initial:                                        3.13          3.08

Volume of NaOH at 1st equivalence pt:       10.0mL       10.2mL

Volumw of NaOh at 2nd equivalence pt:      20.5mL       23.0mL

Questions:

1.Calculate the mg of phosphoric acid in a 6oz cola from the data?

2.Consider the % dissociation for H3PO4 in the initial sample of cola, and the % dissociation of
H2PO4^1- at the 1st equivalence point. Use these two calculated values to explain why Ka1 calculated
using the V1/2 method may not be valid, but Ka2 calculated using this method is valid. Is this
consistent with your experimental values for Ka1 and Ka2, when compared with the literature
accepted values? What general conditions must be true for the V1/2 method to be a valid way of
calculating any weak acid Ka?

Please help this makes no sense to me!!!!

Explanation / Answer

Moles of NaOH added = (0.01 M)(0.01 L) = 0.0001 moles

0.0001 moles H3PO4 are initially present in solution.

0.0001 molesH3PO4*(98grams/1mole)*(1000mg/1gram) = 9.81mg H3PO4 in 20 mL

6 ounces is 180 mL.

9.81mg(180/20) = 88.3mg H3PO4

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