Information beforehand:

Question

mass of weighing vial + NaCl / g	16.2597
mass of “empty” weighing vial / g	14.8136
mass of NaCl used / g	
Final volume of solution / L	0.2500

Questions:

1) Calculate the concentration of the NaCl solution (in mol L-1 and with the correct number of significant digits).

2) Why is it desirable to use the same analytical balance to weigh the weighing vial + NaCl and to weigh the “empty” weighing vial?

3) Avoiding spilling the NaCl in the weighing vial is more important than getting all of the NaCl out of the vial.  Why?

4) When transferring the dissolved NaCl from the beaker to the volumetric flask, all the glassware (beaker, stirring rod, funnel) are thoroughly rinsed with deionized water.  Why?

DrBob222 · Accepted Answer

1) Calculate the concentration of the NaCl solution (in mol L-1 and with the correct number of significant digits).
mols NaCl = grams/molar mass = 1.4461/58.4425 = 0.024744
M = mols/L = 0.024744/0.2500 = ? You are allowed 4 significant figures. That number is limited because of the 0.2500 number.

2) Why is it desirable to use the same analytical balance to weigh the weighing vial + NaCl and to weigh the “empty” weighing vial?
VERY good question. Just in case the weights are a little off in calibration, or some dirt is on one of the weights, or the balance was not zeroed properly or balanced properly or whatever, any errors caused by the vial + NaCl you hope will be the same error caused by the vial itself. Whatever error that is, when subtracted, will cancel. For example, let's just say you weighed an empty vial as 2.000 g because the weight was stamped as 2.000 g. Then you place some NaCl in it and the final weight is 2.150. The mass NaCl is 2.150-2.000 = 0.150 grams. Now let's say that the weight was dirty and that 2.000 g weight actually weighed 2.100 g (that's pretty dirty). Then when you added the NaCl the final weight was 2.250 grams. Now when you subtract 2.250 - 2.100 you get 0.150 grams and voila! That speck of dirt didn't mess up your analysis because you know the mass NaCl is 0.150 g in spite of the fact that the weight was dirty.

3) Avoiding spilling the NaCl in the weighing vial is more important than getting all of the NaCl out of the vial. Why?
Spilling the NaCl solid is more likely to lose more salt than if you leave a little in transferring. Actually, you shouldn't lose ANY in transferring because you can always wash that out and add it to the volumetric flask BEFORE make the flask to the mark with distilled water.

4) When transferring the dissolved NaCl from the beaker to the volumetric flask, all the glassware (beaker, stirring rod, funnel) are thoroughly rinsed with deionized water. Why?
You know that all of these utensils have some NaCl on the surface and that surface must be rinsed clean of any NaCl adhering to them.