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The calculations here are pretty straightforward - you will need to know the following for the Ca-Cl compound (these data need to be in two separate tables in your lab report).
For the Mg-O compound, you will need:
To make the calculations, first calculate the weight of the Ca-Cl compound. You will use your final mass (after the heating/weighing cycles) for this. Now: we know the mass of the Ca (we weighed it!), and we know the mass of the Ca-Cl compound; assuming that the the added mass is due solely to the Cl which combined with the Ca, we can calculate the mass of the Cl. So we know the mass of the Ca and the Cl, and we can convert both of these quantities to moles using the respective molar masses, and from here we can find the ratio of Ca:Cl in the compond (just divide by the smaller of the two mol quantities.)
The calculations for the Mg-O compound are similar: we know the mass of the Mg (~0.5 g), and the mass of the final Mg-O compound; we can find the mass of the oxygen which combined with the Mg, and hence the empirical formula by finding the mol ratio of Mg:O.
Error Analysis:
In this experiment, you know what the results should be based on the ionic charges in the Ca-Cl and Mg-O compounds. If your experimental results do not agree, be prepared to explain in detail why they do not. For example, let us assume that the the Ca-Cl compound was heated too much and some product was lost: where would this affect the results? Conversely, what if the compound was not heated sufficiently to drive off the moisture, and the final mass was too large? Where would this show up in the calculation? Similarly, for the Mg-O compound, what if the Mg was not heated sufficiently? Where would this be manifest in the calculations?
The Ch 205 Web Staff