Find the pH of each mixture of acids. a. 0.075 M in HNO3 and 0.175 M in HC7 H5 O2 b. 0.020 M in

Find the pH of each mixture of acids. a. 0.075 M in HNO3 and...

Find the $\mathrm{pH}$ of each mixture of acids. a. $0.075 \mathrm{M}$ in $\mathrm{HNO}_{3}$ and $0.175 \mathrm{M}$ in $\mathrm{HC}_{7} \mathrm{H}_{5} \mathrm{O}_{2}$ b. $0.020 \mathrm{M}$ in $\mathrm{HBr}$ and $0.015 \mathrm{M}$ in $\mathrm{HClO}_{4}$ c. $0.095 \mathrm{M}$ in $\mathrm{HF}$ and $0.225 \mathrm{M}$ in $\mathrm{HC}_{6} \mathrm{H}_{5} \mathrm{O}$ d. $0.100 \mathrm{M}$ in formic acid and $0.050 \mathrm{M}$ in hypochlorous acid

Find the $\mathrm{pH}$ of each mixture of acids.
a. $0.075 \mathrm{M}$ in $\mathrm{HNO}_{3}$ and $0.175 \mathrm{M}$ in $\mathrm{HC}_{7} \mathrm{H}_{5} \mathrm{O}_{2}$
b. $0.020 \mathrm{M}$ in $\mathrm{HBr}$ and $0.015 \mathrm{M}$ in $\mathrm{HClO}_{4}$
c. $0.095 \mathrm{M}$ in $\mathrm{HF}$ and $0.225 \mathrm{M}$ in $\mathrm{HC}_{6} \mathrm{H}_{5} \mathrm{O}$
d. $0.100 \mathrm{M}$ in formic acid and $0.050 \mathrm{M}$ in hypochlorous acid

Key Concepts

Acid Strength and Dissociation

This concept distinguishes strong acids, which completely ionize in aqueous solutions, from weak acids, which only partially ionize. Understanding the degree of dissociation is crucial because it directly affects the concentration of hydrogen ions (H?) in the solution and, consequently, the pH.

Hydrogen Ion Concentration and pH Calculation

pH is a measure of the acidity of a solution and is defined as the negative logarithm of the hydrogen ion concentration (pH = -log[H?]). Calculating the pH involves accurately determining the total [H?] present, which in mixtures requires summing the contributions from all acid sources.

Weak Acid Equilibrium and Acid Dissociation Constant (K?)

Weak acids do not completely dissociate in water, and their extent of ionization is governed by an equilibrium described by the acid dissociation constant (K?). This equilibrium must be considered when calculating the resulting hydrogen ion concentration, as it will only contribute a fraction of the initial acid concentration to the overall [H?].

Mixture of Acids and Summation of Contributions

When dealing with mixtures of acids, it is important to sum the individual contributions to the hydrogen ion concentration from each acid. For strong acids, the contribution is approximately equal to their initial concentration, while for weak acids, the contribution is determined by their equilibrium dissociation. The overall pH of the mixture is then found by applying the pH = -log[H?] relationship.

Transcript

00:01 When determining the ph of a solution that contains a mixture of acids, if we have two strong acids, the ph is determined by the sum of the two concentrations.

00:11 If we have a strong acid and a weak acid, the ph is determined just by the concentration of the strong acid.

00:21 If we have a mixture of two weak acids, then the ph is determined solely based on the stronger of the two weak acids.

00:31 So for part a we have nitric acid and we have hc7 h502.

00:40 Hc7 h5 .02 is a weak acid.

00:43 So the ph is determined solely based upon the concentration of the strong acid, nitric acid.

00:50 Nitric acid has a concentration of .075 molar.

00:54 Therefore that is the hydronium concentration, which we can take the negative log out to determine ph 1 .12.

01:01 1 -2.

01:04 For the next one, we have a solution that contains hydrobromic acid and perchloric acid.

01:11 In this case, they are both strong acids.

01:14 So to determine the hydronium concentration, we need to sum these two concentrations together.

01:22 When summing the two concentrations, we can then take the negative log of the summed value, which is the hydronium concentration, to get the ph.

01:31 1 .46.

01:33 For the next one, we have phenolic acid and hydrofluic acid, and of the two, you can go to the back of the book and look up the k -a values.

01:44 You'll notice that the k -a value for hydrofluic acid is greater, therefore it is the stronger of the two weak acids, and we will calculate the ph solely based upon its presence.

01:57 To calculate the ph of a solution that contains a weak acid, we need to use that at k -a value that we looked up to compare the two acids, the k -a value is going to be equal to the hydrogenum concentration squared divided by the concentration of hydrofluoric acid minus the hydrogenium concentration.

02:17 Oftentimes, we'll make the assumption that this hydrogenium concentration is small with respect to .095, or less than 5 percent of .095...

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