15 ml of .325 M NaOH is delivered into a 35 ml HObr solution of unknown concentration.The pH of the final solution is measured to be 7.48.Ka for HOBr is 2.0 * 10^-9 at 25C.
Find the number of moles of OBr- and HOBr in the final solution
I don't know how much you are to assume on this and how much you are to work out. Let's first assume what we need.
Since we don't know the (HOBr) at the beginning, we don't know if the final solution is before the equivalence point, at the eq. point, or after the eq. point. Let's assume we are before the eq. point. That means this is a buffered solution and we use the Henderson-Hasselbalch equation.
15 mL x 0.325M NaOH = 4.875 millimoles.
...........NaOH + HOBr ==> NaOBr + H2O
initial..4.875.....?.........0........0
change..-4.875....-4.875..+4.875..+4.875
equil......0......?-4.875..4.875..4.875
pH = pKa + log(base)/(acid)
7.48 = 8.70 + log[4.875/(acid)]
Solve for (acid). It is approximately 80 but you should do it more accurately.
The answer to the problem is you have 4.875 millimoles OBr^- and 80 mmoles HOBr(remember these are approximate) in the final solution and you should convert to moles because the problem asks for moles.
This leaves out the rest of the problem; i.e., how do we know this isn't the eq. point (calculated differently) or past the eq. point (calculated differently). If you don't need to worry about that we can drop it. If you are expected to know how to determine that, you will need to follow up with this question if you don't know how to do it.
To find the number of moles of OBr- and HOBr in the final solution, we need to use the information given and apply some calculations.
First, let's determine the concentration of HOBr in the 35 ml solution before the addition of NaOH. We can use the formula:
M1V1 = M2V2
Where:
M1 = initial concentration of HOBr
V1 = initial volume of HOBr solution
M2 = final concentration of HOBr (which is unknown)
V2 = final volume of the solution after the addition of NaOH
Plugging in the values:
M1 = unknown (let's call it x) M
V1 = 35 ml
M2 = x M
V2 = (35 ml + 15 ml) = 50 ml
Using the formula, we get:
(x M)(35 ml) = (unknown)(50 ml)
(x M)(35 ml) = (unknown)(50 ml)
(35x) = (50)(unknown)
Now, let's calculate the number of moles of HOBr in the final solution:
moles of HOBr = concentration of HOBr × volume of HOBr solution
Given that the final volume of the solution is 50 ml and the concentration of HOBr is x M (which we need to determine), we can calculate:
moles of HOBr = (unknown) × (50 ml)
Next, let's consider the reaction between NaOH and HOBr:
NaOH + HOBr -> NaOBr + H2O
In this reaction, NaOH reacts with HOBr to form NaOBr and water (H2O). Based on stoichiometry, one mole of NaOH reacts with one mole of HOBr to produce one mole of NaOBr.
Since the initial volume of NaOH is 15 ml and its concentration is 0.325 M, we can calculate the number of moles of NaOH:
moles of NaOH = concentration of NaOH × volume of NaOH solution
moles of NaOH = 0.325 M × 15 ml
Now, let's calculate the moles of NaOBr formed:
moles of NaOBr = moles of NaOH
Since one mole of NaOH reacts with one mole of NaOBr, the moles of NaOBr formed will be equal to the moles of NaOH.
Finally, to find the number of moles of OBr- in the final solution, we subtract the moles of NaOBr formed from the moles of HOBr:
moles of OBr- = moles of HOBr - moles of NaOBr
Therefore, to determine the number of moles of OBr- and HOBr in the final solution, follow these steps:
1. Calculate the concentration of HOBr (x M) using the equation (35x) = (50)(unknown).
2. Calculate the moles of HOBr using the equation moles of HOBr = (unknown) × (50 ml).
3. Calculate the moles of NaOH using the equation moles of NaOH = 0.325 M × 15 ml.
4. Calculate the moles of NaOBr using the equation moles of NaOBr = moles of NaOH.
5. Calculate the moles of OBr- using the equation moles of OBr- = moles of HOBr - moles of NaOBr.