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Having just spoken to my teacher about the creation of a standard solution, and she suggested one way of doing it, however a previous chemistry teacher had told us a different way of doing it. My question is, (of the below methods) which is the "correct" way of doing it, and what are the pros/cons of doing one over the other? If you're interested in the full procedure of making this/any other standard solution (with the names of compounds, it is page 24 here).

Method 1:

The solid was weighed accurately in a weighing bottle, and then transferred to the volumetric flask, and then the bottle is re-weighed to calculate the amount of solid transferred to the flask.

Method 2:

Rather than re-weighing the solid in the weighing bottle, the weighing bottle is rinsed with DI water, and then poured into the volumetric flask. (this is repeated a few times).

If I'm not including enough detail in the above methods please let me know and I'll try to expand. For reference the 2 methods are Part 1, step 3 of the above link.

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  • $\begingroup$ It somewhat depends what you're weighing and how soluble it is. $\endgroup$ – NotEvans. May 26 '17 at 18:31
  • $\begingroup$ If working with small volumes and masses, I usually weigh the solid directly in to the volumetric flask. Otherwise, I would go with method 2. But I feel that this is one of those things that is very much a matter of opinion. If you think about it, the "leftover" solid on your weigh boat is probably going to be in the microgram range, and may or may not be significant. Really depends on the purpose of the standard. $\endgroup$ – Bob May 28 '17 at 20:08
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The reason you are using a weighing glass is because it is a lot smaller and it has a much smaller mass than a volumetric flask. These glasses usually are more stable, less affected by temperature, and pressure changes, or drafts in the laboratory. The smaller surface also tends to not accumulate much dirt due to static, your hands, etc.

If you were using method one, then you could skip the part with the weighing glass altogether. This method suffers from getting a stable reading as due to the size it is more affected by the conditions in the laboratory.
Method two is certainly the better method, because the total error tends to be much smaller. When you are using such equipment, then your concentrations are typically smaller than 1 mol/L and you are working with small quantities of solid substances. Continuously rinsing out the weighing vessel will ensure to transfer almost all of your solute.
If you were to produce a saturated solution, then the mass of the solute would not matter in the first place.

If your compound does not readily dissolve then the attempted concentration is too high to be used for analytical chemistry and you should rather start from a saturated solution. All primary standards are easily dissolved. If you are not dissolving a primary standard, then your should check the concentration against one.

Therefore, method one can be used if accuracy is of a minor concern (leave out the weighing glass) because it has to be determined again with a primary standard. Method two should be used when preparing accurately concentrated solutions of primary standards.

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  • $\begingroup$ This is rather interesting as when I approached my teacher she described method 2 as the "quick way" and we are also told to do method 1, not method 2, but thank you :) $\endgroup$ – Crafter0800 Jun 2 '17 at 17:55

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