A 11 mL sample of a solution of AlBr3 was diluted with water to 23 mL. A 15 mL sample of the dilute solution was found to contain 0.045 moles of Br-. What was the concentration of AlBr3 in the original undiluted solution?
answer is 2.09 but i cant get it.
0.045 mols = 45 millimoles which I like better; it keeps all those zeros out of the way.
45 mmoles Br^- x 1/3 = mmoles AlBr3 since there are 3 Br^-/molecule AlBr3. That's 15 mmoles in the 15 mL.
That was the aliquot from 23 mL; therefore 15 mmoles x 23/15 was mmoles in the 23 mL sample. 15 x 23/15 = 23 mmoles (you don't even need a calculator so far).
So we had 23 mmoles in the original 11 mL. M = mmoles/mL = 23/11 = 2.09M
Kate, I think I worked a problem for you earlier that looked a lot like this. If so I must have not read it carefully, for I don't think I approached it this way. If so be sure and check that answer versus this problem.
To solve this problem, we need to use the concept of moles and concentration.
First, we need to calculate the moles of Br- in the 15 mL sample. We are given that there are 0.045 moles of Br- in this portion of the dilute solution.
Next, we need to find the concentration of Br- in the original undiluted solution. To do this, we need to consider the dilution factor, which is the ratio of the final volume to the initial volume. In this case, the dilution factor is 23 mL (final volume) divided by 11 mL (initial volume), which simplifies to 2.09 (approximately).
Now, since the moles of Br- remain constant, we can determine the concentration of Br- in the original undiluted solution by dividing the moles by the dilution factor:
Concentration of Br- = (0.045 moles) / 2.09 = 0.021 M
However, we need to find the concentration of AlBr3, not Br-. The formula for AlBr3 suggests that for one mole of AlBr3, we have three moles of Br-. Therefore, the concentration of AlBr3 can be calculated by multiplying the concentration of Br- by 3:
Concentration of AlBr3 = 0.021 M * 3 = 0.063 M
Thus, the concentration of AlBr3 in the original undiluted solution is 0.063 M, which is equivalent to 2.09 M when rounded to two decimal places.