Video Transcript
In the precipitation method, why do we use ashless filter paper in chemical analysis? (A) Because it filters the precipitate from the solution efficiently. (B) Because we know its mass, so we can calculate the mass of the precipitate easily. (C) Because it is affordable. (D) Because it is completely ignited without leaving ash.
Precipitates are formed in some chemical reactions. If two soluble reactants in aqueous solution are mixed together, an insoluble product may appear. The insoluble product in this case is referred to as a precipitate. This question refers to a precipitation method. This is in the context of precipitation gravimetry. This is an analytical technique that uses the formation and mass of a precipitate to determine the mass of an analyte.
To do this type of analysis effectively, we first need to collect, wash, and then dry the precipitate that’s formed in the reaction. By knowing the accurate mass of precipitate, we can find the mass of analyte that it was formed from. So in this type of analysis, the mass of the analyte is determined from the accurate mass of the precipitate formed in experimental procedure.
Usually, the precipitate is collected by first using a filtration technique. The precipitate is separated from the liquid solution by passing it through a filter paper. The precipitate is then washed with distilled water to remove soluble contaminants. If the accurate mass of precipitate obtained is to be determined, it must be absolutely dry before its mass is measured. The damp filter paper containing the damp precipitate can be placed in a drying oven at 120 degrees Celsius for several hours.
Using this approach, the sample would be dried to constant mass. The mass of the dry filter paper plus the mass of the dry precipitate could then be obtained accurately using an analytical balance. The mass of dry precipitate will be found by subtracting the mass of the filter paper, which would have been determined before the analysis. Ashless filter paper may be used instead. This will ignite during the drying process.
Using this approach, the damp ashless filter paper and the damp precipitate will be heated far more strongly in a crucible. It’s important that the mass of the crucible is accurately determined prior to the analysis. After the ashless filter paper has completely ignited and the precipitate is completely dried, all that remains is a dry precipitate with no residue contained inside the crucible.
Upon cooling, the mass of the crucible plus the mass of the pure dry precipitate can be obtained accurately using analytical balance. The mass of the precipitate is obtained by subtracting the mass of the crucible from the mass determined here. If the filter paper were not truly ashless and it left an ash residue mixed in with the precipitate, we would record a larger mass than expected for the precipitate alone at the end of this procedure. It would therefore be very difficult to obtain the true mass of precipitate formed in the analysis.
Ashless filter paper is used because it leaves no ash residue that could contaminate the dry precipitate sample and therefore affect its mass in the analysis. So to answer the question “In the precipitation method, why do we use ashless filter paper in chemical analysis?” the answer is because it’s completely ignited without leaving ash.
