Calculate the ph of a 0.031 m strong acid solution.

Q: What is the pH of a 0.031 M solution of HCl?

All right, So lets go ahead and Put that concentration in here, zero 31 and grab our calculators, we will take the log of it first and then reverse the sign giving us a ph Of 1.51.

Video Transcript

Hi there. In this question, we have a solution of hcl hydrochloric acid And we are told that its concentration is zero 31 molar hcl. Well, hcl is one of these strong assets and when we use the term strong or weak with acids, what it refers to is the amount of dissociation. When it's placed into water, hydrochloric acid being a strong acid means that it dissociates essentially completely. So when we place this into water as an acquis solution, it is going to pretty much break 100% into ions hydrogen ion and the chloride ion. The polarity of the original acid we're told was .031 Mueller. Well, if all of it dissociates into ions, that means the concentration of each of the ions in solution Is .031 Molar as well. And it's the hydrogen ion that I'm interested in. So, the hydrogen ion concentration, that's what makes it an acid. Is the hydrogen ion. This .031 Mueller. Well, we want to determine the ph the equation that allows us to determine ph from the hydrogen ion concentration is ph is equal to the negative log of that H plus concentration. All right, So let's go ahead and Put that concentration in here, zero 31 and grab our calculators, we will take the log of it first and then reverse the sign giving us a ph Of 1.51. Since our polarity had two significant figures, we need two decimal places when we determine the logarithms. So 1.51 here's the ph All right. Thank you so much for watching, and I hope this information was helpful.

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Video Transcript
Acids and Bases: Calculating pH of a Strong Acid
Calculate the pH of a Hydrobromic Acid (HBr) Solution
pH Question
Key Takeaways: pH of a Weak Acid
pH of a Weak Acid Problem
Solution
Solution: Quick and Dirty Method to Find Weak Acid pH
What is the pH of a 0.031 M solution of HCl?
What is the pH of a 0.025 M HCl solution?
What is the pH of a 0.035 M HCl strong acid solution?

Explanation:

Hydrochloric acid is a strong acid that completely dissociates in water.

For a 0.025 M HCl solution the concentration of the hydrogen ions, #["H"^+]#, is 0.025 moles/liter (i.e. 0.025 M).

pH is defined as #-log ["H"^+]#.

Substituting 0.025 into the equation for pH we find that the 0.025 M HCl solution has a pH of -log(0.025) = 1.60.

Acids and Bases: Calculating pH of a Strong Acid

Calculate the pH of a Hydrobromic Acid (HBr) Solution

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Updated on November 14, 2019

A strong acid is one that completely dissociates into its ions in water. This makes calculating the hydrogen ion concentration, which is the basis of pH, easier than for weak acids. Here's an example of how to determine the pH of a strong acid.

pH Question

What is the pH of a 0.025 M solution of hydrobromic acid (HBr)?

Solution to the Problem

Hydrobromic Acid or HBr is a strong acid and will dissociate completely in water to H+ and Br-. For every mole of HBr, there will be 1 mole of H+, so the concentration of H+ will be the same as the concentration of HBr. Therefore, [H+] = 0.025 M.

pH is calculated by the formula

pH = - log [H+]

To solve the problem, enter the concentration of the hydrogen ion.

pH = - log (0.025)
pH = -(-1.602)
pH = 1.602

Answer

The pH of a 0.025 M solution of Hydrobromic Acid is 1.602.

One quick check to make sure your answer is reasonable is to verify the pH is closer to 1 than to 7 (certainly not higher than this.) Acids have a low pH value. Strong acids typically range in pH from 1 to 3.

Calculating the pH of a weak acid is a bit more complicated than determining the pH of a strong acid because weak acids don't completely dissociate in water. Fortunately, the formula for calculating pH is simple. Here's what you do.

Key Takeaways: pH of a Weak Acid

Finding the pH of a weak acid is a bit more complicated than finding pH of a strong acid because the acid does not fully dissociate into its ions.
The pH equation is still the same (pH = -log[H+]), but you need to use the acid dissociation constant (Ka) to find [H+].
There are two main methods of solving for hydrogen ion concentration. One involves the quadratic equation. The other assumes the weak acid barely dissociates in water and approximates the pH. Which one you choose depends on how accurate you need the answer to be. For homework, use the quadratic equation. For a quick estimate in the lab, use the approximation.

pH of a Weak Acid Problem

What is the pH of a 0.01 M benzoic acid solution?

Given: benzoic acid Ka= 6.5 x 10-5

Solution

Benzoic acid dissociates in water as:

C6H5COOH → H+ + C6H5COO-

The formula for Ka is:

Ka = [H+][B-]/[HB]

where:
[H+] = concentration of H+ ions
[B-] = concentration of conjugate base ions
[HB] = concentration of undissociated acid molecules
for a reaction HB → H+ + B-

Benzoic acid dissociates one H+ ion for every C6H5COO- ion, so [H+] = [C6H5COO-].

Let x represent the concentration of H+ that dissociates from HB, then [HB] = C - x where C is the initial concentration.

Enter these values into the Ka equation:

Ka = x · x / (C -x)
Ka = x²/(C - x)
(C - x)Ka = x²
x² = CKa - xKa
x² + Kax - CKa = 0

Solve for x using the quadratic equation:

x = [-b ± (b² - 4ac)½]/2a

x = [-Ka + (Ka² + 4CKa)½]/2

**Note** Technically, there are two solutions for x. Since x represents a concentration of ions in solution, the value for x cannot be negative.

Enter values for Ka and C:

Ka = 6.5 x 10-5
C = 0.01 M

x = {-6.5 x 10-5 + [(6.5 x 10-5)² + 4(0.01)(6.5 x 10-5)]½}/2
x = (-6.5 x 10-5 + 1.6 x 10-3)/2
x = (1.5 x 10-3)/2
x = 7.7 x 10-4

Find pH:

pH = -log[H+]

pH = -log(x)
pH = -log(7.7 x 10-4)
pH = -(-3.11)
pH = 3.11

Answer

The pH of a 0.01 M benzoic acid solution is 3.11.

Solution: Quick and Dirty Method to Find Weak Acid pH

Most weak acids barely dissociate in solution. In this solution we found the acid only dissociated by 7.7 x 10-4 M. The original concentration was 1 x 10-2 or 770 times stronger than the dissociated ion concentration.

Values for C - x then, would be very close to C to seem unchanged. If we substitute C for (C - x) in the Ka equation,

Ka = x²/(C - x)
Ka = x²/C

With this, there is no need to use the quadratic equation to solve for x:

x² = Ka·C

x² = (6.5 x 10-5)(0.01)
x² = 6.5 x 10-7
x = 8.06 x 10-4

Find pH

pH = -log[H+]

pH = -log(x)
pH = -log(8.06 x 10-4)
pH = -(-3.09)
pH = 3.09

Note the two answers are nearly identical with only 0.02 difference. Also notice the difference between the first method's x and the second method's x is only 0.000036 M. For most laboratory situations, the second method is "good enough" and much simpler.

Check your work before reporting a value. The pH of a weak acid should be less than 7 (not neutral) and it's usually less than the value for a strong acid. Note there are exceptions. For example, the pH of hydrochloric acid is 3.01 for a 1 mM solution, while the pH of hydrofluoric acid is also low, with a value of 3.27 for a 1 mM solution.

Sources

Bates, Roger G. (1973). Determination of pH: theory and practice. Wiley.
Covington, A. K.; Bates, R. G.; Durst, R. A. (1985). "Definitions of pH scales, standard reference values, measurement of pH, and related terminology". Pure Appl. Chem. 57 (3): 531–542. doi:10.1351/pac198557030531
Housecroft, C. E.; Sharpe, A. G. (2004). Inorganic Chemistry (2nd ed.). Prentice Hall. ISBN 978-0130399137.
Myers, Rollie J. (2010). "One-Hundred Years of pH". Journal of Chemical Education. 87 (1): 30–32. doi:10.1021/ed800002c
Miessler G. L.; Tarr D .A. (1998). Inorganic Chemistry (2nd ed.). Prentice-Hall. ISBN 0-13-841891-8.

What is the pH of a 0.031 M solution of HCl?

All right, So let's go ahead and Put that concentration in here, zero 31 and grab our calculators, we will take the log of it first and then reverse the sign giving us a ph Of 1.51.