[bdub21]

So I've finished all of the experiment and now I'm just trying to get my calculations correct... But for some reason they're not coming out right. What I did was dissolve two different weights of an unknown solid into two separate flasks filled with a known amount of DI water. This was then titrated (after the solution was passed through a resin column) with NaOH. I am supposed to find out the percent by weight of the Ca+2 ions in the solution. Here are my calculations, but they're not reading as correct when entered into the computer grading program for this experiment.


Converting amount of NaOH used to titrate into moles using the known molarity of the NaOH solution:
19.48 mL NaOH * (1 L / 1000 mL) * (0.2343 mol NaOH / 1 L NaOH) = 0.004564 mol NaOH

Determining the moles of H+ ions present:
0.004564 mol NaOH * (1 mol H+ / 1 mol NaOH) = 0.004564 mol H+

Determining the moles of Ca+2 ions present:
0.004564 mol H+ * (1 mol Ca+2 / 2 mol H+) = 0.002282 mol Ca+2

Detemining the amount of Ca+2 in grams (it was said in the experiment to assume the weight molecular unknown was 200 grams per mol Ca+2):
0.002282 mol Ca+2 * (200 g Ca+2 / 1 mol Ca+2) = 0.456416 g Ca+2

Calculating the fractional weight of the unknown used (25.00 mL was pipetted out of the 75.00 mL solution this particular portion of the unknown was dissolved in):
2.0970 g Unknown Total * (1 / 3) of Unknown Solution Used = 0.699 g Unknown Used

Calculation for percent by weight Ca+2 ions:
(0.456416 g Ca+2 / 0.699 g Unknown) * 100% = 65.2956% Ca+2 ions


Can someone tell me where I'm going wrong? A quick reply is appreciated!

[alysha27]

"Detemining the amount of Ca+2 in grams (it was said in the experiment to assume the weight molecular unknown was 200 grams per mol Ca+2):
0.002282 mol Ca+2 * (200 g Ca+2 / 1 mol Ca+2) = 0.456416 g Ca+2"

I believe this is the step where you went wrong. u don't have 200 g of Ca2+ per mol. You have 40.078 g of Ca2+ per mol of Ca2+. Don't use the weight of the molecular unknown unless you are at a step where u are converting to the number of grams of the whole compound