Exploring Chemical Analysis
Exploring Chemical Analysis
5th Edition
ISBN: 9781429275033
Author: Daniel C. Harris
Publisher: Macmillan Higher Education
Question
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Chapter 9, Problem 9.10P

(a)

Interpretation Introduction

Interpretation:

The pH of solution formed from solution of glycine amide hydrochloride (BH+) and glycine amide (B) has to be calculated.

Concept Introduction:

The equation for buffer is described by Henderson-Hasselbach equation and it is a rearranged form of equilibrium constant, Ka. It relates pH of buffer solution and pKa value. It also helps to find equilibrium pH in acid-base reactions. Consider an equation of dissociation of an acid as follows:

  HAH++A

Formula to calculate pH is as follows:

  pH=pKa+log([A][HA])

Here,

pKa is acid dissociation constant of weak acid HA.

[A] is concentration of conjugate base A

[HA] is concentration of acid HA.

Concentration of a solution is expressed by molarity and is denoted by M. It is defined as the ratio of number of moles of solute to volume of solution in 1 L.

The formula to calculate moles is as follows:

  Moles=mass of solutemolar mass of solute

The formula used to calculate concentration is as follows:

  Concentration=molesvolume of solution(L)

(a)

Expert Solution
Check Mark

Answer to Problem 9.10P

pH of solution is 3.59.

Explanation of Solution

The formula to calculate concentration of glycine amide (B) is as follows:

  [B]=(mass of Bmolar mass B)volume of solution(L)        (1)

Substitute 1.00 g for mass of B, 74.08 g/mol for molar mass of B and 0.100 L for volume of solution in equation (1).

  [B]=(1.00 g74.08 g/mol)0.100 L=0.135 M

The formula to calculate concentration of glycine amide hydrochloride (BH+) is as follows:

  [BH+]=(mass of BH+molar mass BH+)volume of solution(L)        (2)

Substitute 1.00 g for mass of BH+, 110.54 g/mol for molar mass of BH+ and 0.100 L for volume of solution in equation (2).

  [BH+]=(1.00 g110.54 g/mol)0.100 L=0.090 M

Hence, concentration of glycine amide (B) is 0.135 M and glycine amide hydrochloride (BH+) is 0.090 M. In accordance to Henderson-Hasselbach equation pH is calculated as follows:

  pH=pKa+log([B][BH+])        (3)

Here,

pKa is acid dissociation constant.

[B] is concentration of B.

[BH+] is concentration of BH+.

Substitute 8.20 for pKa, 0.090 M for [BH+] and 0.135 M for [B] in equation (3).

  pH=8.20+log(0.135 M0.090 M)=8.37

Hence, pH of solution is 8.37.

(b)

Interpretation Introduction

Interpretation:

Mass of glycine amide (B) at pH equals to 8 has to be calculated.

Concept Introduction:

Refer to part (a).

(b)

Expert Solution
Check Mark

Answer to Problem 9.10P

Mass of glycine amide (B) is 0.423 g.

Explanation of Solution

The formula to calculate concentration of glycine amide hydrochloride (BH+) is as follows:

  [BH+]=(mass of BH+molar mass BH+)volume of solution(L)        (2)

Substitute 1.00 g for mass of BH+, 110.54 g/mol for molar mass of BH+ and 100 mL for volume of solution in equation (2).

  [BH+]=(1.00 g110.54 g/mol)100 mL(1 mL103 L)=0.090 M

In accordance to Henderson-Hasselbach equation pH is calculated as follows:

  pH=pKa+log([B][BH+])        (4)

Rearrange equation (4) for [B].

  [B]=10(pHpKa+log[BH+])        (5)

Here,

pKa is acid dissociation constant.

[B] is concentration of B.

[BH+] is concentration of BH+.

Substitute 8.20 for pKa, 0.090 M for [BH+] and 8 for pH in equation (3).

  [B]=10(88.20+log[0.090 M] )=101.25=0.057 M

The formula to calculate concentration of glycine amide (B) is as follows:

  [B]=(mass of Bmolar mass B)volume of solution(L)        (1)

Rearrange equation (1) for mass of B.

  Mass of B=[B](volume of solution(L))(molar mass of B)        (6)

Substitute 0.057 M for [B], 74.08 g/mol for molar mass of B and 100 mL for volume of solution in equation (6).

  Mass of B=(0.0572 M)(74.08 g/mol)(100 mL)(103 L1 mL)=0.423 g

Hence, mass of glycine amide (B) is 0.423 g.

(c)

Interpretation Introduction

Interpretation:

The pH of solution formed from mixture of HCl with solution of glycine amide hydrochloride (BH+) and glycine amide (B) has to be calculated.

Concept Introduction:

Refer to part (a).

(c)

Expert Solution
Check Mark

Answer to Problem 9.10P

pH of solution is 8.33.

Explanation of Solution

The formula to calculate number of moles of glycine amide (B) is as follows:

  Moles=mass of Bmolar mass B        (7)

Substitute 1.00 g for mass of B and 74.08 g/mol for molar mass of in equation (7).

  Moles of B=(1.00 g74.08 g/mol)=0.0135 mol

The formula to calculate number of glycine amide hydrochloride (BH+) is as follows:

  Moles of BH+=mass of BH+molar mass BH+        (8)

Substitute 1.00 g for mass of BH+ and 110.54 g/mol for molar mass of BH+ in equation (8).

  Moles of BH+=1.00 g110.54 g/mol=0.0090 M

The formula used to calculate concentration of HCl is as follows:

  Concentration of HCl=moles of HClvolume of solution(L)        (9)

Rearrange equation (9) for moles of HCl.

  Moles of HCl=(Concentration of HCl)(volume of solution(L))        (10)

Substitute 0.100 M for concentration of HCl and 5 mL for volume of solution in equation (10).

  Moles of HCl=(0.100 M)(5 mL)(103 L1 mL)=0.0005

Solution formed of glycine amide hydrochloride (BH+) and glycine amide (B) forms buffer and addition of HCl produces conjugate acid. B is denoted for base and BH+ for its conjugate acid. Since volume is similar therefore, number of moles is equal to concentration. The ICE table for reaction is as follows:

B+H+BH+Initial concentration(M)0.01350.00050.0090Change in concentration(M)0.0005+0.0005final concentration(M)0.0130.0095

Hence, concentration of B is 0.013 M and its conjugate acid BH+ is 0.0095 M. In accordance to Henderson-Hasselbach equation pH is calculated as follows:

  pH=pKa+log([B][BH+])        (11)

Here,

pKa is acid dissociation constant.

[B] is concentration of B.

[BH+] is concentration of BH+.

Substitute 8.20 for pKa, 0.0095 M for [BH+] and 0.013 M for [B] in equation (11).

  pH=8.20+log(0.013 M0.0095 M)=8.33

Hence, pH of solution is 8.33.

(d)

Interpretation Introduction

Interpretation:

The pH of solution formed from mixture of NaOH with solution of glycine amide hydrochloride (BH+) and glycine amide (B) has to be calculated.

Concept Introduction:

Refer to part (a).

(d)

Expert Solution
Check Mark

Answer to Problem 9.10P

pH of solution is 8.41.

Explanation of Solution

The formula to calculate number of moles of glycine amide (B) is as follows:

  Moles=mass of Bmolar mass B        (7)

Substitute 1.00 g for mass of B and 74.08 g/mol for molar mass of in equation (7).

  Moles of B=(1.00 g74.08 g/mol)=0.0135 mol

The formula to calculate number of glycine amide hydrochloride (BH+) is as follows:

  Moles of BH+=mass of BH+molar mass BH+        (8)

Substitute 1.00 g for mass of BH+ and 110.54 g/mol for molar mass of BH+ in equation (8).

  Moles of BH+=1.00 g110.54 g/mol=0.0090 M

The formula used to calculate concentration of NaOH is as follows:

  Concentration of NaOH=moles of NaOHvolume of solution(L)        (12)

Rearrange equation (12) for moles of NaOH.

  Moles of NaOH=(Concentration of NaOH)(volume of solution(L))        (13)

Substitute 0.100 M for concentration of NaOH and 10 mL for volume of solution in equation (13).

  Moles of HCl=(0.100 M)(10 mL)(103 L1 mL)=0.001

Solution formed of glycine amide hydrochloride (BH+) and glycine amide (B) forms buffer and addition of NaOH produces conjugate acid. Since volume is similar therefore, number of moles is equal to concentration. The ICE table for reaction is as follows:

BH++OHBInitial concentration(M)0.00950.0010.013Change in concentration(M)0.001+0.0010final concentration(M)0.008550.014

Hence, concentration of B is 0.014 M and its conjugate acid BH+ is 0.00855 M. In accordance to Henderson-Hasselbach equation pH is calculated as follows:

  pH=pKa+log([B][BH+])        (14)

Here,

pKa is acid dissociation constant.

[B] is concentration of B.

[BH+] is concentration of BH+.

Substitute 8.20 for pKa, 0.00855 M for [BH+] and 0.014 M for [B] in equation (14).

  pH=8.20+log(0.014 M0.00855 M)=8.41

Hence, pH of solution is 8.41.

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