OBJECTIVE: to find out which of two indicators, methyl orange or phenolphthalein, gives more reliable and consistent results in the titration of H3PO4 with NaOH.

REQUIREMENT: (1). Phosphoric acid solution of unknown concentration. (2). Standard NaOH solution. (3). Methyl orange and phenolphthalein indicators. (4). 5cm^3 pipette, 10cm^3 burette, conical flasks etc.

PROCEDURE: (1). Titrate 5.0ml of the phosphoric acid solution with sodium hydroxide using 2-3 drops of methyl orange as indicator. Record the titre value. (2). AFTER the methyl orange end point, add 2-3 drops of phenolphthalein to the solution and CONTINUE THE TITRATION to the phenophthalein end point. Record this titre value. (3). Repeat the titration one more time. (4). For the third titration, add ONLY 2-3 drops of phenolphthalein to the 5.0ml phosphoric acid solution pipetted, and record the titre value required to reach the end point.

(1). Write the chemical reactions corresponding to the end points with (a) methyl orange (b) phenolphthalein

(2). Calculate the concentration of the phosphoric acid from (a) methyl orange end point (b) phenolphthalein end point

(3). Which result do you consider more reliable, and why?

Have you done the titration yet, and did you make any observations?

Answers please

To determine which indicator (methyl orange or phenolphthalein) gives more reliable and consistent results in the titration of H3PO4 with NaOH, we need to follow a specific procedure outlined above.

1. Chemical reactions corresponding to the end points:
(a) Methyl orange end point:
H3PO4 + 2NaOH → Na2HPO4 + H2O
(Phosphoric acid reacts with sodium hydroxide to form sodium phosphate and water).

(b) Phenolphthalein end point:
Na2HPO4 + 2NaOH → Na3PO4 + 2H2O
(Sodium phosphate reacts with sodium hydroxide to form trisodium phosphate and water).

2. Calculation of the concentration of the phosphoric acid:
(a) Methyl orange end point:
Using the recorded titration values obtained with methyl orange, you can calculate the concentration of the phosphoric acid by using the equation:
Moles of NaOH used = Moles of H3PO4 present in 5.0 mL
Concentration of H3PO4 = Moles of H3PO4 / Volume of H3PO4

(b) Phenolphthalein end point:
Using the recorded titration values obtained with phenolphthalein, you can calculate the concentration of the phosphoric acid by using the equation mentioned above.

3. Determining the more reliable result:
To determine which result is more reliable, you need to consider the consistency of the titration values obtained with each indicator. Look for the titration values that are the most consistent and have the least variation between replicates. A more reliable result is the one that consistently gives similar titration values, indicating the indicator is providing accurate and consistent information about the endpoint of the reaction.

After comparing the data obtained from both indicators, you can analyze the standard deviation or percent relative deviation to see which set of titration values is more consistent and reliable. The smaller the standard deviation or percent relative deviation, the more reliable and precise the indicator is.

Taking into account the consistency and reliability of the titration values obtained, you can conclude which indicator (methyl orange or phenolphthalein) provides more reliable and consistent results in the titration of H3PO4 with NaOH.