What Are the Steps for Weighing Chemicals on Balances?

Types of Mechanical Balances

• Mechanical balances are less accurate and take longer to use than digital balances, but they are much cheaper and are often used in classrooms. The most basic is the dual pan balance, in which two pans are balanced on either side of a fulcrum. Dual pan balances usually come with a set of labeled, unattached weights of various masses. Triple beam balances use a single pan for the sample, and the weights are moved left or right on an adjacent beam.

Using a Dual Pan Balance

• Make sure that the pans are properly balanced while empty. Most balances have an adjustment knob for this purpose. Place the sample in one of the two pans. That pan should lower with the added weight. Next, place various combinations of weights on the opposite pan. At some point, an added weight will shift the balance and the sample pan will rise. Remove the weight you just added and replace it with a smaller weight. The goal is for both pans to be perfectly balanced. Once both pans are balanced, meaning that neither is higher or lower than the other, you know that the sample's mass is equal to the combined mass of the weights on the other pan. If this isn't possible with the weights provided, simply take the average between the heaviest combination of weights which is lighter than the sample and the lightest combination of weights which is heavier than the sample. When the weights are close to being balanced, the pans will move up and down. Wait until they have both stopped moving before writing down any results.

Using a Triple Beam Balance

• The procedure for a triple beam balance is very similar to that of a dual pan, except that the provided weights are attached to the device and are simply slid along a beam which is labeled with different masses according to the distance from the fulcrum. Start by sliding all of the attached weights to "0" positions. Use the adjustment knob to ensure that the indicator needle is neither above nor below the neutral line while the pan is empty. Add the sample to the pan. Move the largest weight along the beam until the balance shifts in favor the weights (meaning the sample pan rises), then slide it back one slot so the sample pan falls again. Repeat with the next largest weight, and then again with the next largest, and so on until the needle points at the neutral line. As with the dual pan balance, wait for any movement to cease before taking a reading.

Using a Digital Balance

• Place either an empty beaker or a piece of weighing paper on the balance. Press the tare button to reset the scale to zero and effectively disregard the weight of the paper or beaker. Carefully add small amounts of sample until the desired weight is reached. Unless the lab procedure dictates otherwise, it should be okay to have a slightly different weight than what is prescribed by the procedure -- just make sure to write down the actual measured weight and use it for your calculations.

Additional Rules for Analytical Balances

• Analytical balances are very precise and therefore susceptible to very small disturbances. Never place any chemical directly on an analytical balance. Do not use weighing paper, because small amounts of material will likely adhere to the paper and lead to errors in your measurement. Instead, use the same beaker or container in which the reaction will occur. Analytical balances are enclosed in clear glass or plastic boxes to avoid disruption by air currents, so make sure that the door is closed when taking a measurement. Use gloves to avoid putting fingerprints on glass -- analytical balances are sensitive enough to weigh the oils from a fingerprint. When weighing materials which have been drying in a desiccator, measure each sample at the same time interval after removal from the desiccator. For example, weigh each sample exactly ten seconds after placing it in the pan -- this will help reduce error due to the sample absorbing water from the air.