posted by morgan on .
a student was asked to prepare 100.0 mL of 0.0200 M solution of HCL from 0.400 M solution of HCL. in order to do this he measured 5.0 mL of 0.400 M HCL with a graduated cylinder, transfered this amount to a 100 mL beaker, added 95 mL of deionized water measured with a graduated cylinder and mixed all components of the resulting solution. explain what basic mistakes were done by the student. what should the student have done to to obtain the final target solution. explain two key mistakes student made
The dilution formula is
M1*V1 = M2*V2
0.02 * 100 = 0.4 * V2
V2 = 2 / 0.4
V2 = 5 mL
The volume of stock solution used is correct. And we need 95 mL of water to make the target solution.
One mistake here is adding water to acid. Note that you should NOT add WATER to ACID, it should always be ACID to WATER. Dilution of acid (water to acid) is extremely exothermic, that it may result to boiling and spattering.
Another mistake that I see is the glassware used. You don't make solutions in graduated cylinders. You make solutions in volumetric flasks because they are more accurate in volume.
*In the second one, I mean you don't usually make solutions of defined volume and concentration in beakers, because these glasswares are not accurate in volume. (sorry about this, I misread ^^;)
Also, using volumetric pipettes or simply pipettes are more accurate than graduated cylinders.
And note that, before you add the 5 mL acid to the 100-mL volumetric flask, add some amount of water, of course, below the 100 mL mark. (water -> acid -> water)
hope this helps :3