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What is the pH of the resulting solution when equal volumes of 0.1M NaOH and 0.01M HCl are mixed ?
A.7.0
B.1.04
C.12.65
D.2.0

Answer
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Hint: To calculate pH first of all we have to find concentration of H+ ion also pH=log[H+] and to calculate pOH we have to calculate concentration of OH ion in the resulting solution . We should remember the formula pH+pOH=14.

Complete step by step answer:
 Chemical reaction for the given solution is : .
Let us assume volume of both NaOH and HCl is equal to 1L then we have 0.1 moles of NaOH and 0.01 moles of HCl by using the formula n=M×Vwhere n is number of moles , M is molarity and Vis volume of the solution .
Since we have 0.1 mole of NaOH and only 0.01 mole of HCl , so only 0.01 mole of OH ion will take part in neutralisation reaction .
Now number of moles of OH ion =0.10.01=0.09
Total volume of solution =2L
Concentration of OH =0.092 =0.045M
pOH=log[OH]=log[0.045]
pOH=1.35
Now we know that pH+pOH=14 . So pH of the resulting solution will be equal to 14pOH .
pH=14pOH=141.35
pH=12.65

So option (C) is the correct answer .


Note:
 High pH means that the solution is basic in nature as it is clear from the above question because it has an excess amount of NaOH and its pH value is greater than seven . Similarly the high value of pOH means solution is acidic in nature . Neutral solutions have both pH and pOH of seven . Another point to be noted down is that pH decreases with increase in temperature because as the temperature rises molecular vibrations increases which results in the ability of water to ionise and form more hydrogen ions . As a result pH will drop . But we should not be confused that water becomes more acidic at a higher temperature because a solution is considered to be acidic if there is excess hydrogen ions . In the case of water there is always the same concentration of H+ and OH.
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