Under what conditions does the activity of an ion fn solution most closely appro
ID: 927854 • Letter: U
Question
Under what conditions does the activity of an ion fn solution most closely approach the concentration? concentration and ionic strength are both large concentration and ionic strength are both small concentration is large but ionic strength is small concentration is small but ionic strength is large What volume of 0.100 M MgCl_2 must be diluted to exactly one liter in order to prepare a solution having an ionic strength of 0.0500? Often one uses a few milliliters of indicator solution for certain titrations (starch for iodometric methods or 2,7-dichlorofluorescein for Fajan's method) but only a few drops of indicator (methyl red or phenolphthalein) for neutralization titrations. The reason for the difference in amount of indicator used is that neutralization indicator solutions are much more concentrated. the pH range over which neutralization indicators change color is much shorter neutralization indicators are titrated whereas the others do not alter the stoichiometry. most titrations, other, than neutralization reactions, are performed in carefully buffered solutions.Explanation / Answer
47. Activity, a = c. .....[=activity coefficient]
from Debye Huckel theory, log = -Az2I .....[I= ionic strength = 1/2 iciz2i]
For dilute solution c<<1 and I<<1
So = 10-Az2I = 1 ....[when ionic strength is very low for dilute solution]
For dilute solution a=c
Ans: B
48. If concentration of the salt in final solution is m(M),
I= 0.05 = 0.5[m.22 + 2m*12] = 3m
m = 0.0167 M
Volume of MgCl2 have to be diluted
= 0.0167 mol.L-1 *1000 mL/0.1 m.L-1
= 166 mL
49. Neutralization indicators are also called acid-base indicators. They change color in certain pH range. To detect the equivalence point of an acid base reaction pH at the equivalence point must be in the pH range of the indicator. In fact during the acid base titration the neutralization indicators are also titrated. So adding excess amount of indicator can change the stoichiometry of the titration.
But in other titrations, for example in iodometry, iodine dissolved in an aqueous solution of potassium iodide—the triiodide anion (I3) complexes with starch, producing an intense purple color. It does not change the stoichiometry of the titration; it cannot alter the equivalence point.
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