Equal volumes of two solutions of pH 5.0 and 7.0 are mixed together. The pH of the resulting solution is __________.Correct answer is between '5.2,5.4'. Can you explain this answer?

Question

Answer

To get an exact solution to this case, you have to go like this to convert the pH back to concentration units. pH 4 equals a 0.0001 Molar hydronium concentration while pH 6 equals a 0.000001 Molar hydronium concentration. A mixture of the two is half of each or 0.00005 M plus 0.0000005 M or 0.0000505 M. The log of 0.0000505 is -4.2967 and since pH is the inverse of the log this corresponds to pH 4.297. If, instead of mixing equal volumes of the unbuffered pH 4 solution with the unbuffered pH 6 solution, you mixed equal volumes of pH 4 solution and deionized water, the hydronium concentration would be 0.00005, the log of which is -4.301 and the pH would be 4.301. As you can see, dilution with the pH 6 solution gives a nearly the same pH 4.297 as the pH 4.301 seen when diluting with water.

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