Using the total volume at endpoint (51.98mL) and the fact that the concentration of CrO4 ^2- due to Mohr's indicator is 0.26 M, find the actual concentration of CrO4^ 2- at the endpoint (or at the instant just before Ag2CrO4 begins to form).
Any help I am completely lost with this one
How much chromate did you use? If you used, for example, 2 mL, then the concn chromate will be 0.26 x 2 mL/52 mL = ??
the only chromate used was the amount present in 1mL of Mohr's indicator. Does it matter if no addition chromate was added?
There is no need to add more chromate than that in the procedure you're followinig. In that case [CrO4^2-] = 0.26M x (1/52) = ?M.(51.98 instead of 52 if you want to split hairs.) Most procedures that I've seen try to keep [CrO4^2-] somewhere between 0.005 and 0.01M.
thanks
To find the actual concentration of CrO4^2- at the endpoint, you need to first understand the principle behind the titration and the role of Mohr's indicator.
In this case, it seems you are performing a titration involving the chromate ion (CrO4^2-) and using Mohr's indicator to determine the endpoint. Mohr's indicator changes color when the concentration of chromate ion reaches a certain level, indicating the endpoint of the titration.
Now, let's break down the problem step by step.
Step 1: Write the balanced chemical equation for the reaction. Based on the information given, it seems you are titrating a solution containing chromate ion (CrO4^2-) with another solution containing Ag+ ions (which will react with the chromate ion to form Ag2CrO4).
The balanced chemical equation for this reaction is:
2 Ag+ (aq) + CrO4^2- (aq) → Ag2CrO4 (s)
Step 2: Calculate the number of moles of Ag+ ions reacted. Since the concentration of Ag+ ions is not provided, we can assume it is in excess. Therefore, the number of moles of Ag+ ions used in the reaction will equal the number of moles of Ag2CrO4 formed.
Step 3: Calculate the number of moles of Ag2CrO4 formed. Since Ag2CrO4 is a solid precipitate, its volume can be neglected. Therefore, the number of moles of Ag2CrO4 will be equal to its mass divided by its molar mass.
Step 4: Calculate the number of moles of CrO4^2- reacted. From the balanced chemical equation, we know that the stoichiometric ratio between Ag2CrO4 and CrO4^2- is 1:1. Therefore, the number of moles of CrO4^2- reacted will be equal to the moles of Ag2CrO4.
Step 5: Calculate the concentration of CrO4^2- at the endpoint. The actual concentration of CrO4^2- at the endpoint can be determined by dividing the number of moles of CrO4^2- reacted by the total volume at the endpoint.
Given the total volume at the endpoint (51.98 mL) and the concentration of CrO4^2- due to Mohr's indicator (0.26 M), you can now calculate the actual concentration of CrO4^2- at the endpoint.