Illustrated Guide to Home Chemistry Experiments

(Amelia) #1
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
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