Video Transcript
When we mix silver nitrate and
potassium chloride together, a white precipitate of silver chloride forms. Calculate the mass of potassium
chloride when two grams of silver chloride is precipitated. Give your answer to two decimal
places. The molar mass of silver is 108
grams per mole, potassium is 39 grams per mole, and chlorine is 35.5 grams per
mole.
In this question, we are asked to
determine the mass of potassium chloride in a solution that reacted when given the
mass of a silver chloride precipitate. This is an example of precipitation
gravimetry, which is an analytical technique that uses the formation and mass of a
precipitate to determine the mass of an analyte. In this problem, the precipitate is
silver chloride and the analyte is potassium chloride.
Before we can calculate the mass of
potassium chloride, we need to write a balanced equation for the precipitation
reaction. Silver nitrate, or AgNO3, reacts
with potassium chloride, or KCl. And one of the products of the
reaction is the silver chloride precipitate, or AgCl. Based on the equation that we have
so far, we have not included the potassium or nitrate ions in the products. Therefore, the missing product is
potassium nitrate, or KNO3, which remains dissolved in the solution after the
reaction.
After counting up the number of
each type of atom on either side of the equation, we notice that the chemical
equation is already balanced. We now know that the molar ratio of
potassium chloride to silver chloride is one to one. Determining the mass of potassium
chloride that reacted is going to involve three steps. First, we will convert grams of
silver chloride to moles. Then, we will use the molar ratio
from the balanced equation to determine how many moles of KCl reacted. Finally, we will convert moles of
KCl to grams of KCl. We can use the following equation
to calculate the number of moles of silver chloride by dividing the mass in grams by
the molar mass.
The molar mass of silver chloride
is found by adding together the average molar masses of silver and chlorine that
were provided in the problem. The result is 143.5 grams per
mole. Now, we can substitute the mass of
silver chloride, which is two grams, and the molar mass of silver chloride that we
just calculated into our equation. After dividing, we get the number
of moles of AgCl. We can see in our balanced equation
that for every one mole of KCl that reacts, one mole of AgCl is produced. This means that the number of moles
of KCl that reacted is equal to the number of moles of AgCl that was produced.
Now we’re ready to convert the
amount of moles of KCl to grams. To find the mass in grams, we can
use the following equation and multiply the number of moles of KCl by the molar mass
of KCl. The molar mass of KCl is 39 grams
per mole plus 35.5 grams per mole, which is 74.5 grams per mole. Now, we can substitute the values
for moles and molar mass into the equation. After multiplying the number of
moles of KCl by 74.5 grams per mole, we finally have the mass in grams of KCl. Now, we must round our answer to
two decimal places. This gives us 1.04 grams.
In conclusion, the mass of
potassium chloride that reacts with silver nitrate to produce two grams of silver
chloride precipitate is 1.04 grams.
