Advance Study Assignment: pH, Its Measurement and Applications A solution of a w

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Advance Study Assignment: pH, Its Measurement and Applications A solution of a weak acid was tested with the indicators used in this experiment. The colors observed were as follows: Methyl violet Congo red orange-red Thymol blue yellow Bromcresol green green Methyl yellow yellow What is the approximate pH of the solution? ______ A solution of Na_2CO_3, has a pH of 10. The CO_3^2- ion is the conjugate base of the HCO_3^-, ion, Write the net ionic equation for the reaction which makes a solution of Na_2CO_3, basic. (Eq.7.) The pH of a 0.10 M HCN solution is 5.2. What is [H^+] in that solution? What is [CN^-]? what is [HCN]? (Where do the H and CN^- ions come from?) ______ M What is the value of K_a, for HCN? What is the value of pk_a, ? ______ M; ______ M Formic acid, H For, has a K_a, value equal to about 1.8 times 10^-4. A student is asked to prepare a buffer having a pH of 3.4 from a solution of formic acid and a solution of sodium formate having the same molarity. How many milliliters of the Na For solution should she add to 20 ml of the H For solution to make the buffer? (See discussion of buffers.) ______ mL How many ml. of 0.10 M N_2OH should the student add lo 20 ml 0.10 M H For if she wished to prepare a buffer with a pH of 3.4, the same as in Problem 4? ______ mL

Explanation / Answer

Question 1

The pH for a weak acid solution was tested using 5 indicators. First, we need to see at what pH the given indicators show the given color.

So, we can estimate the pH of the solution to be approximate to 5.4 (we're talking about a weak acid so this pH is normal for weak acids), however I would suggest using another indicator like methyl red which is yellow at pH > 6.2 and orange between 4.4 and 6.2. If the solution turns orange then we can me more sure that the pH of the solution is close to 5.4 (and if it turns yellow, that indicates the pH is more than 6.2 and we would need to experiment using more indicators).

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