The pH of an HCHO 2 /NaCHO 2 solution is 3.11. The pKa of HCHO 2 is 3.74. Which

ID: 868930 • Letter: T

Question

The pH of an HCHO2/NaCHO2 solution is 3.11. The pKa of HCHO2 is 3.74. Which of the following is TRUE?

A) [HCHO2] < [NaCHO2]

B) [HCHO2] = [NaCHO2]

C) [HCHO2] > [NaCHO2]

D) It is not possible to make a buffer of this pH from HCHO2 and NaCHO2.

Consider the pKa values provided below:

HF= 3.5

HC7H5O2 = 4.2 (benzoic acid)

HClO= 7.5

HCN= 9.3

Which one of the following provides the best mole ratio for the conjugate base to acid, respectively, to create a buffer with a pH of 8.1?

A) 4.0 NaF: HF

B) 3.9 NaC7H5O2: HC7H5O2

C) 4.0 NaClO: HClO

D) 6.3 NaCN: HCN

Please show with steps!

The pH of an HCHO2/NaCHO2 solution is 3.11. The pKa of HCHO2 is 3.74. Which of the following is TRUE?

3,11 =3,74 + log [NaCHO2]/[HCHO2]

[NaCHO2]/[HCHO2]= 0,23

[NaCHO2]= 0,23[HCHO2]

Supossing that [HCHO2] is 1M then the concentration of [NaCHO2] its going to be 0,23M so the option is the C.

2) The ideal buffer is that has the pKa +/- 1 order near the pH that wants to be prepared. This makes the regular capacity to be better.

So if you see the responses the HF and HC7H5O2 have a pKa to low.

Now the HClO and HCN can be used, using the Enderson-Hasselback equation we can see wich ratio is better.

pH= pKa + log [A-]/[HA]

HClO ---- 8,1 = 7,5 + log [A-]/[HA] ----- [A-]/[HA]= 100,6= 3,98

HCN ----- 8,1 = 9,3 + log [A-]/[HA] ------ [A-]/[HA]= 10-1,2= 0,063

Looking at the answer we see that the ratio of HCN its wrongto get the pH that we want, so the correct answer is 4,0 NaClO:HClO

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