Civil Engineering - GATE Exam Questions - Discussion

3. 

A wastewater sample contains 10-5.6 mmol/l of OH ions at 25° C. The pH of this sample is

[A]. 8.6
[B]. 8.4
[C]. 5.6
[D]. 5.4

Answer: Option D

Explanation:

No answer description available for this question.

Rathi said: (May 29, 2014)  
OH- concentration : 10-5.6 mmol/l = 10-8.6 mol/l.

H+ concentration : 10-14-(-8.6) = 10-5.4 mol/l.

Therefore pH value : 5.4.

Gantela Gandhi Prasad said: (Dec 8, 2014)  
Your calculation will be wrong 10^(-8.6) you have to replace 10^(-8.4).

Angiela said: (Nov 18, 2015)  
How can we compute?

Pran said: (Sep 17, 2016)  
The answer is 8.4.

=> 14 - 5.6 = 8.4.

Rakesh Kumar said: (Jan 13, 2017)  
Which is correct answer? Please tell me.

Bhawna said: (Jan 27, 2017)  
D is correct because - 5.6mmol/l convert in mol/l =-8.6.
Then 14 - 8.6 = 5.4.

Chhaya said: (Jul 4, 2017)  
Yes, @Rathi & @Bhawan is right.

1mol= 1000 mmol.
10^-5.6+10^-3=10^-8.6.

Sarvesh Saxsena said: (Jul 9, 2017)  
Ans is D, but you are wrong @Chhaya.
Yes, 1 mol=1000mmol.
So,10^-5.6 mmol=10^(-5.6) 10^(-3) =10^(-8.6).
therfore,POH=8.6 & PH= 14-8.6=5.4.

Bhanu said: (Oct 13, 2017)  
Answer will 8.4 also because in qus didn't mentioned units of P^H.

Sheela said: (Dec 20, 2017)  
@ Bhanu:

pH is the negative of the base 10 logarithms of the molar concentration, measured in units of moles per liter, of hydrogen ions. To attain standard unit here mmol/L is converted into mol/L.

Bharath said: (Jan 16, 2018)  
@Sheela :
pH = -log10^-5.6-3 = -log10^-8.6 = 8.6.
then, 14-8.6 = 5.4.

B.S.Dalal said: (Oct 16, 2018)  
Ph -log ^-5.6=-log10^-8.6 = (14-8.6) = 5.4.

Dhanu said: (Jan 9, 2019)  
How come 14 here?

Ibrahim said: (Jan 10, 2019)  
PH + POH = 14.

Since we are given a concentration of Oh ions we are calculating POH. To get PH we subtract from 14.

Dhanu said: (Jan 21, 2019)  
Thank you all for explaining this.

Suhail said: (Oct 1, 2020)  
Thanks all for explaining it.

Manjula said: (May 1, 2021)  
H+ * OH- = 10^-14.
H+ *(10^-5.6 *10^-3) = 10^-14.
H+= (10^-14)/(10^-8.6)
= 10^-5.4.

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