Measurement: Mass

Measuring Mass

The primary method that you will use to measure the amounts of chemicals is to weigh them--that is, to determine their mass. To do this you use a balance, specifically an electronic top-loading balance.

General Rules for Using Balances

Let's go over some of the general rules for using balances.

Do not place chemicals directly on the balance pan unless they are inert (unreactive) and at room temperature. Containers, glassware, or pieces of metal can generally be put directly on the balance pan, but for most other chemicals, you need to use a container for the chemicals. (The containers, of course, are chemicals, but they are inert.) The reason for this rule is that many chemicals react with the balance pan. Some attract moisture and cause corrosion. Any residue that sticks to the pan can interfere with weighing. It could also contaminate any other chemicals weighed later in the same way. So, instead of weighing materials directly on the balance pan, always weigh the chemicals in or on something--a weighing dish, a beaker, or a piece of folded paper. When you do, remember to weigh the container first or adjust for its mass.

Be careful not to spill any chemicals on or around the balance. If you do, clean up immediately!

If for some reason your balance doesn't seem to be adjusted properly, let the instructor know. Don't just try to fix it yourself. Tell us, so we can deal with it and get it functioning properly.

When you're finished with the balance, re-zero it. Then it will be ready for the next person to use it.

Electronic Top-Loading Balances

The usual operation of an electronic top-loading balance is quite simple. This type of balance can weigh things to three decimal places; you may encounter a slightly different balance in the lab that weighs to a different number of places, but the basic operation is always the same. It should be left on at all times. When you're through using it, leave it on. If the components of an electronic balance are cold when you start, they will drift while warming up, causing changes in your measurements. Leaving it on so it stays warm will avoid this.

Steps to Operate

Start by placing a container on the balance pan. Close the doors, then press the TARE or ZERO button. That sets the balance to weigh anything that is placed into the container, but it does not weigh the container itself.
When "0.000" shows in the display, it is time to add what you want to weigh to the dish. Open the doors, add, and re-close. It is important that you close the doors when taking your reading.
Wait for the value displayed to stabilize, then record the mass of the material in the container.
Remove the container, clean up any spills, reset the balance to zero, and close the cover.

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