136 DIY Science: Illustrated Guide to Home Chemistry Experiments
SBSTITUTIU oNS ANd modIfICATIoNS- You may substitute a 100 mL graduated cylinder for
the 100 mL volumetric flask, with some loss in accuracy. - You may substitute any convenient mixing container of
the appropriate size for the 150 mL beaker.
LABoRAToRy 7.3:
mAkE Up A moLAR SoLUTIoN of A LIqUId CHEmICAL
Some common laboratory chemicals, such as
sulfuric acid, are liquid at standard temperature
and pressure. Many other chemicals, such as
ammonia and hydrochloric acid, are gases, but
are ordinarily stored and handled in the form of
concentrated aqueous solutions. Whether the
chemical is actually a liquid or is supplied in the
form of a concentrated aqueous solution, the
same principles apply to diluting the chemical
to make up working solutions of known
molarities.
RIREEqU d EqUIpmENT ANd SUppLIES£ goggles, gloves, and protective clothing£ balance and weighing papers£ volumetric flask, 100 mL£ graduated cylinder, 10 mL£ beaker, 150 mL£ funnel£ eye dropper£ wash bottle (distilled or deionized water)£ labeled storage bottle£ hydrochloric acid, 37% (8.33 mL)In this laboratory, we make up 100 mL of 1.00 M hydrochloric
acid, which is a common bench solution. (I actually make
this stuff up 500 mL at a time, but not everyone has a 500
mL volumetric flask.) We’ll make up this solution by diluting
concentrated hydrochloric acid. The label on the bottle of
reagent-grade hydrochloric acid gives us several pieces of
information about its concentration:
- The mass percentage is specified as 36% to 38%, which
means that 100 g of the concentrated acid contains between
36g and 38 g of dissolved HCl gas. - The density is specified as ranging between 1.179 g/mL and
1.189 g/mL, which tells us the mass of a specific volume of the
concentrated acid. (The density is also specified as between
22° and 23° Baumé, which is an older and obsolescent method
for specifying density or specific gravity.) - The molarity is specified as between 11.64 M and 12.39 M.
If they were mixing up a 1.0 M bench solution, most chemists
would assume that the concentrated hydrochloric acid was
12.0 M, and add one part of concentrated acid to 11 parts water
to make up the 1 M solution. To make up 100 mL of 1 M acid, we
could measure 8.33 mL of the 12 M acid, add it to 70 mL or so
of distilled water in a 100 mL volumetric flask, and make up the
solution to 100 mL. In practice, that’s how it’s usually done.
But there are advantages to using mass percentage and density
when making up dilute bench solutions, including higher accuracy
and precision. Also, it’s important to know how to make up
solutions based on mass percentage and density, because not all
concentrated aqueous solutions list a molarity.
To make up solutions based on the mass percentage of a stock
solution, begin by calculating the required number of moles of
solute. For 100 mL (0.1 L) of 1 M hydrochloric acid, we need 0.1
moles of HCl. The formula weight of HCl is 36.46 g/mol, so we
need 3.646 g of HCl. If the mass percentage of the concentrated
acid is 37% (0.37), that means we need 3.646/0.37 = 9.854 g of
the concentrated acid. To get that, we tare a small beaker or other
container on our balance, and add concentrated acid to the beaker
until the balance reads 9.854 g.If we prefer to measure the concentrated acid volumetrically
rather than gravimetrically, we also need to know the density
of the solution to determine how many mL of the concentrated
acid has a mass of 9.854 g. To calculate that value, we divide the
required mass by the density of the solution. If the concentrated
acid has a density of 1.183 g/mL, that means we need
9.854/1.183 = 8.330 mL.