A 25.00 mL sample of a soft drink was titrated using 12.10 mL of 1.50 M NaOH. Given that the soft drink's monoprotic acid has a Ka value of 2.8 x 10^-5, predict the pH of the soft drink.
I know that the answer is pH 2.85 (our practice questions have an answer key) but I'm really struggling to understand how to do the question. If anyone could help I would be super grateful. Thanks!
Call the soft drink, since it is monoprotic, HA.
Then HA + NaOH ==> NaA + H2O
mols NaOH = M x L = 0.0121 x 1.5 = ?
mols HA = the same because of the 1:1 ratio in the balanced equation.
M HA = mols HA/L HA or M HA = approx 0.7 but yu need a better answer than that estimate. Then
........HA ==> H^+ + A^-
I......0.7M....0.....0
C......-x......x.....x
E....0.7-x.....x.....x
Ka = 2.8E-5 = (x)(x)/(0.7-x)
Solve for x and convert to pH. I worked through it and 2.85 for pH is right.
To determine the pH of the soft drink, we need to understand the concepts of titration and acid-base reactions.
In this case, the soft drink contains a monoprotic acid, which means it can donate only one proton (H+ ion) when it reacts with a base. The base being used here is sodium hydroxide (NaOH), which dissociates completely in water to produce sodium ions (Na+) and hydroxide ions (OH-).
The balanced equation for the reaction between the acid in the soft drink and NaOH can be written as follows:
Monoprotic acid + NaOH → Salt + Water
Since the volume and concentration of NaOH used during titration are known, we can use this information to calculate how many moles of NaOH were consumed. The stoichiometric ratio between the acid and base will allow us to determine the number of moles of acid present in the soft drink.
1. Calculate the number of moles of NaOH:
moles of NaOH = concentration of NaOH (in M) × volume of NaOH (in L)
= 1.50 M × 0.01210 L
2. Determine the number of moles of acid:
Since the acid and NaOH have a 1:1 stoichiometric ratio, the number of moles of acid will be equal to the number of moles of NaOH from step 1.
3. Calculate the concentration of the acid:
We know that the volume of the soft drink is 25.00 mL, which can be converted to liters (L).
concentration of acid (in M) = moles of acid ÷ volume of soft drink (in L)
= moles of acid ÷ 0.02500 L
4. Calculate the concentration of H+ ions:
Since the acid in the soft drink is monoprotic, the concentration of H+ ions will be the same as the concentration of the acid.
Now that we have determined the concentration of H+ ions, we can calculate the pH using the definition of pH:
pH = -log[H+]
Substituting the concentration of H+ ions into this equation will give us the pH of the soft drink.
Given that the monoprotic acid in the soft drink has a Ka value of 2.8 x 10^-5, we can infer that it is a weak acid. In this case, the concentration of H+ ions is less than the initial concentration of the acid due to the reaction with NaOH. Thus, to complete the calculation accurately, we need to consider the change in concentration of the acid by comparing the initial concentration with the amount consumed.
By following these steps, we can determine the pH of the soft drink to be approximately 2.85.