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Question

# Which of the following solution will have a pH exactly equal to 9 ?

A
109 M HCl solution at 250C
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B
2×103 M Ba(OH)2 solution at 250C
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C
109 M NaOH solution at 250C
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D
None of the above
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Solution

## The correct option is D None of the above(a) 10−9 M HCl is a very dilute solution where water dissociation needs to be considered to calculate the total [H+] ions present in the solution. [H+] from HCl = 10−9 M [H+] from H2O = 10−7 M Total [H+]=(10−9+10−7)=1.01×10−7 M We know that pH=−log10[H+] pH=−log10(1.01×10−7)=6.99. (b) Ba(OH)2 is a strong base and in 1 liter aqueous solution 1 mol Ba(OH)2 dissociates to give 2 mol OH− ions. ∴[OH−]=4×10−3M⇒[H+][OH−]=10−14⇒[H+]=2.5×10−12M⇒pH=−log10[H+]=−log10(2.5×10−12)=11.60 (c) [OH−] from H2O = 10−7 M Total [OH−]=(10−9+10−7)=1.01×10−7 M We know that pOH=−log10[OH−] pOH=−log10(1.01×10−7)=6.99pH+pOH=14pH=14−6.99=7.01 Therefore none of these options are correct.

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