Weighing a solid on a balance

Solid reagents are typically added into a tared weighing boat, folded filter paper or foil with a spatula before moving onto the balance. This is to avoid spilling hazardous material onto the balance area, possibly contaminating subsequent uses. Remember to tare or zero the balance with the weighing vessel prior to measuring out material. Where clean containers (eg singe use weighing boats) are used, any excess reagent may be returned to the container during the weighing process.

There are two types of balances available in teaching labs, ones that measures to 4 decimal places, and others which measures to 2 decimal places. Along with the difference in precision for weighing, there is also a difference in the maximum mass the different balances can measure. Typically 4 d.p. balances are only able to weigh a combined mass (reagent and vessel) of a few hundered grams, whereas the 2 d.p. balances may be able to weigh several kilogrammes.

As a general rule, 2 d.p. balance can be used when weighing reagents that are in excess, or the scale of reaction is such that weighing to a precision greater than 0.1 g would not be important. Whereas 4 d.p. balances shold generally be used for limiting reagents where more than 1 d.p. would be required to give at least 3 significant figures of precision in the weighing.

When weighing, it is not generally nescessary (or advisable) to weigh precisely the amount calculated, but to accurately weigh an amount close to this value. The measured mass should be somewhere near the desired value so we are not changing the stoichiometry, but does not need to be precisely the calculated quantity. However, the actual quantity taken should be recorded in your lab book, and used to determine theoretical maximum yields etc.

When weighing, it is possible that some material may be inadvertently spilled, either on the work bench or on the balance. This is not a problem as long as it is cleaned up  before recording a final mass, and certainly before the balance is used by the next user. Try and be considerate as the balances are a shared resource and should be kept in good condition! Material left on balances can get inside the mechanisms and easily damages the balances, which in turn affects their accuracy in weighing.

Material may be brushed off the balance onto the bench, and then wiped from the bench with tissue and solvent as appropriate. Demonstrators can assist you to help clean up spills.