Question

Incorrect Question 11 0/1 pts A 25.0 mL sample of a solution of a monoprotic acid is titrated with a 0.115 M NaOH solution. The titration curve below was obtained. Which of the following indicators would be best for this titration? 12.01 10.0 8.0 pH of solution in flask 6.01 Equivalence Point Indicator methyl orange methyl red bromocresol purple bromthymol blue thymol blue phenolpthalein pka 3.46 5.00 6.12 7.10 8.90 9.10 5 10 15 20 25 30 35 40 45 mL of 0.115 M NaOH added to flask phenolpthalein bromthymol blue thymol blue bromocresol purple methyl red
Please explain the correct answer to all three questions. Thank you!

Answer 1

Solution11. The titration curve clearly shows that the monoprotic acid used is a very weak acid, as the rise in pH near end-point is not very sharp. The weaker the acid, the slower it will be in absorbing the OH- ions supplied by the NaOH.

Near the end-point the pH will be greater than 7. The indicator used should be such that is color change occur at above 7.

This is true for Thymol blue as well as phenolphthalein.

However, with thymol blue, the range is between 8.0 to 9.6 ( color change from yellow to blue). This might not give the exact end point. The equivalence point might occur slightly above 8 also. This cannot be detected with thymol blue.

With Phenolphthalein, the range is 8.2 to 10. The color change would be from colorless to red. This would be best indicator in this case. Option a)

Solution 12. Calcium Phosphate would exist in the following equilibrium:

                   Ca₃(PO₄)_2(s)$\rightleftharpoons$ 3 Ca²⁺_(aq) + 2PO₄^3-_(aq)

Now, understand that addition of acid (H⁺) ions would react with phosphate ions as:

PO₄^3- + H⁺$\rightleftharpoons$ HPO₄^2-      ... and also other reactions are possible with HPO₄^2-

The main point is that phosphate ions are getting consumed by addition of acid. If the equilibrium of calcium phosphate is thus disturbed, it will try to re-establish itself (Le-Chatelier's principle) by producing more phophate ions. The solubility of Calcium phospahte would therefore increase as the equilibrium shift towards right.

Thus option b) is correct

Solution 13. All the processes (except the last one) mentioned are spontaneous process. It is because they do not need any outside intervention.

For example, you do not need to stir the cup of hot milk to dissolve the sugar.(we do it only to make the process faster!, else, it will happen on its own in some time).

A raw banana left on its own will ripen itself.

In Nitrogen formation, entropy is decreased, but the enthalpy of the reaction, due to very stable nitrogen molecule with three triple bonds, the reaction has negative free energy. Thus it is spontaneous.

Similarly, Lightning is formation of NO from nitrogen and Oxygen at very high temperatures. Note that fortunately, this combination does not take place at normal temperature and pressure.

The last one, that is, Formation of CH4 and O2 from CO2 and H2O however needs energy.

Note that spontaneous does not mean that reaction occur rapidly.

Kindly submit feedback if explanation is satisfactory.