Video Transcript
Which of the following choices
shows the correct numerical order of the steps of the experiment of quantitative
analysis using the precipitation method? (1) Mix the substances together to
yield the precipitate. (2) Transfer the precipitate to an
ignition crucible and completely ignite. (3) Isolate the precipitate by
using ashless filter paper. (4) Weigh the mass of the remaining
precipitate. (A) 1, 2, 3, 4; (B) 1, 2, 4, 3; (C)
1, 4, 3, 2; (D) 1, 3, 2, 4; (E) 1, 4, 2, 3.
This question is asking us about
the steps of a laboratory procedure that would be performed during quantitative
chemical analysis using the precipitation method. The type of quantitative analysis
that uses a precipitation reaction to determine the mass of an analyte is called
precipitation gravimetry. Let’s clear some space to discuss a
possible sequence of steps used in this type of experiment.
In a precipitation reaction, a
reaction between two soluble substances in solution produces an insoluble solid
product called a precipitate. So, the first step in the
experiment would be to mix together the soluble substances in the two solutions to
form a precipitate. This corresponds to step one in the
provided list of steps.
Next, we need to isolate the
precipitate from the rest of the solution so that we can eventually determine its
mass. The precipitate can be separated
from the rest of the solution by using filtration. It’s important that ashless filter
paper is used during this step. So, step two of the experiment is
to isolate the precipitate by using ashless filter paper.
Now, the problem is that the filter
paper and the precipitate are wet. The precipitate needs to be
completely dry in order for us to analyze its mass. We can fold the filter paper over
the wet precipitate and place it in an ignition crucible, which is typically made of
porcelain or metal. This allows us to heat the filter
paper strongly enough so that it ignites. At first the paper will dry, but
then it will begin to char.
After continuing to heat even more
strongly, at some point, all of the paper will have burned. With all of the remnants of the
ashless filter paper gone, only the dry precipitate remains in the crucible. So, step three of the experiment is
to transfer the precipitate to an ignition crucible and completely ignite.
Now that the precipitate is fully
dry, we can find its mass using an analytical balance.
Now let’s bring the answer choices
back up on screen to answer the question. Through our discussion, we have
determined that the correct numerical order of the steps listed in the problem is 1,
3, 2, 4. Therefore, the correct answer is
answer choice (D): 1, 3, 2, 4.
