# Civil Engineering - GATE Exam Questions - Discussion

### Discussion :: GATE Exam Questions - Section 2 (Q.No.3)

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

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.

 Shashank Pandit said: (Sep 16, 2022) poh = 8.6. As the mmol will change to mole per lit; therefore as we know; pH + pOH = 14. pH + 8.6 = 14. pH = 14 - 8.6 = 5.4.