Illustrated Guide to Home Chemistry Experiments

Chapter 5: Mastering Laboratory Skills 73

USING A BALANCE

An accurate balance is essential for doing good science in a home lab, but even the best balance

must be used and maintained properly if it is to provide reliable results. Here are some guidelines

for using and maintaining your balance:

• ays read and follow the instructions supplied with your Alw
  balance. Balances are delicate instruments that can be
  damaged easily if you use or maintain them improperly.


• Make sure that the balance is level. Many balances read
  inaccurately if they are even slightly out of level.


• Set up the balance on the “dry” side of your lab or workbench.
  Although you may sometimes use the balance to weigh liquids,
  it should be kept well away from the sink and other areas where
  large quantities of liquids are used and spills are likely.


• Know the maximum capacity of your balance and do not
  exceed it. Most electronic and mechanical balances are
  relatively well protected against small overloads, but it’s easy
  to damage or destroy some balances by placing a gross
  overload on the balance pan.


• Cover the balance when it is not in use. If no dust cover
  was supplied with the balance, purchase a dust cover
  separately or use a plastic bag or a cardboard box of
  the appropriate size.

Measuring Liquids by Volume ..........................................................................................................

It’s often convenient to measure liquids by mass rather

than by volume. For example, if you need 0.1 mole of

hydrochloric acid for a reaction, you could calculate the

volume of concentrated (37%, 12 M) hydrochloric acid

solution that contains 0.1 mole of the acid and measure

that volume of the concentrated acid using a graduated

cylinder. Alternatively, because you know that one gram

of 37% hydrochloric acid contains 0.37 gram of the acid

and that the gram-molecular mass of hydrochloric acid

is about 36.5 g/mol, it’s easy to calculate the amount

you need by mass. You need 0.1 mole of acid, or 3.65 g.

The concentrated acid contains 0.37 g of acid per gram

of solution, so you need (3.65 g)/(0.37 g/mL) = 9.85 g of

concentrated acid solution.

To obtain that mass of concentrated hydrochloric acid

solution, place a small beaker or similar container on

the balance pan, tare the balance to read 0.00 g with the

beaker in place, and use a pipette to transfer 9.85 g of

concentrated hydrochloric acid solution to the beaker.

• ate the balance periodically by using it to weigh a Calibr
  known mass. Many balances are supplied with calibration
  weights. If yours was not, purchase an inexpensive set of
  calibration weights that covers the range of the balance.
  Such sets are available from most laboratory equipment
  vendors.


• Never place the sample directly on the balance pan. Use a
  weighing paper for small samples or a weighing boat for larger
  or bulky samples. Although you can buy weighing papers, a
  10 centimeter square of ordinary waxed paper works just as
  well. A plastic cup or similar container works well as a weighing
  boat, as shown in Figure 5-5.


• If you have an electronic balance, learn to use the tare function
  (rhymes with air), which zeros out the display when the
  balance pan already has some mass on it. For example, placing
  a weighing boat on the balance pan may cause the balance to
  read 8.47 g. Pressing the tare button (sometimes labeled 0.00,
  zero, Z, or T) resets the display to read 0.00 g, which means
  that the subsequent reading reflects only the mass of the
  sample that you add to the weighing boat.

FIGURE 5-5:

Using a weighing boat to measure the mass of a bulky solid sample