<Li> Secondly, the precipitate is converted to a more chemically stable form . For instance, calcium ion might be precipitated using oxalate ion, to produce calcium oxalate (CaC O); it might then be heated to convert it into the oxide (CaO). It is vital that the empirical formula of the weighed precipitate be known, and that the precipitate be pure; if two forms are present, the results will be inaccurate . </Li> <Li> The precipitate cannot be weighed with the necessary accuracy in place on the filter paper; nor can the precipitate be completely removed from the filter paper in order to weigh it . The precipitate can be carefully heated in a crucible until the filter paper has burned away; this leaves only the precipitate . (As the name suggests, "ashless" paper is used so that the precipitate is not contaminated with ash .) </Li> <Li> After the precipitate is allowed to cool (preferably in a desiccator to keep it from absorbing moisture), it is weighed (in the crucible). To calculate the final mass of the analyte, the starting mass of the empty crucible is subtracted from the final mass of the crucible containing the sample . Since the composition of the precipitate is known, it is simple to calculate the mass of analyte in the original sample . </Li> <P> A chunk of ore is to be analyzed for sulfur content . It is treated with concentrated nitric acid and potassium chlorate to convert all of the sulfur to sulfate (SO 2 − 4). The nitrate and chlorate are removed by treating the solution with concentrated HCl . The sulfate is precipitated with barium (Ba) and weighed as BaSO . </P>

Describe one method of determination of phase sequence