Measurements to Make Solutions

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Measurements to Make Solutions

Postby rita_debbaneh2G » Sun Oct 25, 2020 10:21 am

I was reviewing Fundamental E and came across this question in the textbook that asked "Why do chemists typically not measure out very precise, predetermined masses when making up solutions?" I'm just wondering if anyone knows the answer to this since I'd like to understand this topic more conceptually.

Zaid Bustami 1B
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Re: Measurements to Make Solutions

Postby Zaid Bustami 1B » Sun Oct 25, 2020 1:57 pm

Hi Rita,
To the best of my knowledge, when chemists make solutions, there's a lot of room for error when adding dry solute. Some solute can get stuck to the scoopula you use to scoop up the solute, some can get stuck on the sides of the beaker, some can get spilled while transferring the solute. To account for this, chemists will add a rough estimate of the amount of solute needed into a volumetric flask, then measure the mass of added solute using a scale. Once that mass is recorded, water is added until the water level reaches the indication on the flask for the desired volume (e.g. 100mL for a 100mL volumetric flask). You can guess now that the mass is converted to moles and concentration is determined by dividing that by the volume of the volumetric flask.
It's far more practical to guesstimate than to add a predetermined mass of solute, because you avoid all of those sources of error I mentioned previously. This way you measure all the solute that actually ended up in the flask rather than the amount you think was added.
Also, if chemists want to get an exact concentration of solute, rather than make the exact concentration each time (which takes time) chemists take from a stock solution of known concentration (one that was probably guesstimated) and dilute it or mix it with a more concentrated stock solution (remember adding dry solute will factor in those errors I mentioned). In order to know how much water or more concentrated solution needs to be added, the M1V1 = M2V2 equation is used.
Hope that answered your question!

Natalie Do 3F
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Joined: Wed Sep 30, 2020 10:03 pm

Re: Measurements to Make Solutions

Postby Natalie Do 3F » Tue Dec 15, 2020 4:46 pm

Just a TLDR of Zaid's great response

There is a lot of human error and chemistry happens on a very small scale so measuring precisely can be counterintuitive when maybe a couple of drops of the solution gets stuck to the side of the cup or splatter onto the table.

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