4. During a practical exercise, you find out that you need to adjust the pH of a strong acidic solution of volume 5.6 litres from 4.52 to 5.25
(i) What would you add, OH- or H3O+ to adjust the pH? Explain your answer.
(ii) Assuming that there is a negligible volume change, how many moles of OH- or H3O+ must be added to bring about the adjustment.
i. From 4.52 to 5.25 is going to more basic solution so you will need to add >>>>>>>?
ii. Convert pH 4.52 to H^+. Convert pH 5.25 to H^+. Subtract to find the difference (which will be in units of moles/L) and multiply that difference by 5.6 L (that's the volume you have to work with). That will give you moles of what must be added.
1.4 * 10^-4 moles OH-
To adjust the pH of a strong acidic solution, you would need to add OH- ions. Here's why:
(i) The pH scale ranges from 0 to 14, with 7 being neutral. When the pH is less than 7, it indicates an acidic solution. A pH value of 4.52 is lower than 5.25, which means the solution is more acidic than the desired pH. To increase the pH and make the solution less acidic, you need to add a base.
OH- ions are the hydroxide ions, which are the conjugate base of water (H2O). When OH- ions react with the hydronium ions (H3O+) in an acidic solution, they form water (H2O). This reaction reduces the concentration of H3O+ ions, resulting in an increase in pH or alkalinity.
(ii) To determine the number of moles of OH- or H3O+ ions needed to bring about the adjustment, you need to use the balanced chemical equation of the neutralization reaction between H3O+ and OH-.
Since there is a negligible volume change, we can assume the moles of H3O+ initially present are equal to the moles required to neutralize the OH- ions added.
First, calculate the initial concentration of H3O+ (c1):
c1 = 10^(-pH) = 10^(-4.52) M
Second, calculate the desired concentration of H3O+ (c2):
c2 = 10^(-pH) = 10^(-5.25) M
The change in concentration (Δc) of H3O+ can be calculated as:
Δc = c2 - c1 = 10^(-5.25) - 10^(-4.52) M
Since the volume of the solution is constant at 5.6 liters, the moles of OH- (or H3O+) required are given by:
moles = Δc x volume = (Δc) x (5.6 L)
By substituting the calculated values, you can determine the number of moles of OH- (or H3O+) needed to adjust the pH of the solution.