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?

I need your help please

1) H3PO4 + 3NaOH --> Na3PO4 + 3H2O

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(1).

(a) Chemical reaction corresponding to the end point with methyl orange:

H3PO4 + 3NaOH → Na3PO4 + 3H2O

Methyl orange is an acid-base indicator that changes color in the pH range of 3.1 to 4.4. At the end point, when all the acid has reacted with the base, the pH of the solution is neutral, indicating the completion of the titration.

(b) Chemical reaction corresponding to the end point with phenolphthalein:

H3PO4 + 2NaOH → Na2HPO4 + 2H2O

Phenolphthalein is also an acid-base indicator, but it changes color in the pH range of 8.2 to 10.0. At the end point, when all the acid has reacted with the base, the pH of the solution becomes basic, indicating the completion of the titration.

(2).

(a) Calculation of the concentration of phosphoric acid from the methyl orange end point:

The concentration of phosphoric acid can be calculated using the formula:

Concentration (M) = (Volume of NaOH (L) * Molarity of NaOH (M)) / Volume of H3PO4 (L)

(b) Calculation of the concentration of phosphoric acid from the phenolphthalein end point:

The concentration of phosphoric acid can be calculated using the same formula as above.

(3).

To determine which result is more reliable, we need to consider the characteristics of the indicators and the consistency of the results obtained.

Methyl orange has a more narrow pH range for its color change and is typically used for titrations involving strong acids and strong bases. It is more sensitive to small changes in pH and can give more accurate results for strongly acidic or basic solutions. However, it may not be suitable for titrations involving weak acids or weak bases, as the pH change may not be distinct and accurate.

Phenolphthalein, on the other hand, has a wider pH range for its color change and is typically used for titrations involving weak acids and weak bases. It is less sensitive to small changes in pH and may not give accurate results for strongly acidic or basic solutions.

In this case, since we are titrating phosphoric acid with sodium hydroxide, which are both moderately strong, either indicator can give reliable results. However, based on the provided procedure, it seems that the phenolphthalein end point (in step 2) is closer to the expected neutral pH (indicating the completion of the titration) compared to the methyl orange end point (in step 1).

Therefore, the result obtained from the phenolphthalein titration (step 2) can be considered more reliable and consistent in this particular experiment.