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Can someone please explain how to avoid errors in Gravimetric Estimation?

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There are a few methods that you need to follow properly to avoid errors. As I never recommend a rote learning approach, I will think through these by going through the chronological process of the gravimetric analysis:

Precipitation: The precipitation reaction can go wrong in two ways (overestimate or underestimate):

Firstly, the reaction may not reach 100% - this can be avoided by adding an excess amount of the precipitant to ensure that the reaction proceeds as much as possible. If this is not done, the analysis will underestimate the content you are trying to measure (as less of it will become precipitated and measured)

Secondly, there may be unintended reactions (sometimes referred to as "cross-reactions") - this is hard to avoid if you are following instructions on a prac sheet, but the lab technician needs to design the prac properly so they are not going to precipitate out other chemicals that you are not trying to measure. This will obviously overestimate the content you are trying to measure (as you will weigh more stuff, but it is not what you are trying to measure..)

Filtration: The filtration process can go wrong in two ways again:

Precipitate can leak through the filter paper if the vacuum pump is too strong, or if the filter paper is not applied properly (wet the filter paper onto the funnel). This can be noticed if cloudy substances fall into the filtrate (the solution that passes through the filter). This will lead to an underestimate of the amount measured.

Precipitate can also NOT go through the filter paper if the precipitate and solution mixture is not thoroughly rinsed with water. If this is not done, soluble ions may end up remaining above the filter, and will add to the weight of the precipitate (just like how when saltwater dries out, salt reappears, even though it was soluble!). This will lead to an overestimate of the amount measured.

Drying: The main error that occurs here is "underdrying". When you place the wet filter paper in the oven, you need to "heat to a constant mass", to indicate that it is fully dried. If you simply trust it is dry after the first attempt, there may still be water, leading to added mass (overestimate). This can be avoided by measuring each oven attempt, and repeating the oven process until the weight is stable - this indicates the water is fully evaporated.

Hope that helps!