Video Transcript
During an experiment in a
laboratory, Daniel carefully weighs out 20 grams of an alkali called sodium
hydroxide and dissolves it in 200 milliliters of warm water. Chloe takes 50 grams of sodium
hydroxide from the same jar and dissolves it in 400 milliliters of warm water. Finally, Sophia takes five grams of
sodium hydroxide from the same jar and dissolves it in 100 milliliters of warm
water. Which student has made the most
concentrated alkali solution?
The question mentions sodium
hydroxide. Sodium hydroxide is a white, solid
substance. It has the chemical formula
NaOH. Sodium hydroxide is made from
positively charged sodium ions and negatively charged hydroxide ions. Hydroxide ions contain oxygen and
hydrogen, and together they have a negative charge. Sodium hydroxide breaks apart, or
we say dissociates, into sodium ions and hydroxide ions when it is added to
water. We are told that sodium hydroxide
is an alkali. An alkali is a substance that
dissociates, or breaks apart, in water, producing negative hydroxide ions. Alkalis turn red litmus paper
blue.
We are told that three students
weigh some sodium hydroxide from a jar and dissolve it in water. Each student weighs a different
amount of sodium hydroxide and dissolves their sodium hydroxide in a different
amount of water. Daniel weighs out 20 grams and
dissolves it in 200 milliliters of warm water. Chloe weighs out 50 grams and
dissolves this in 400 milliliters of warm water. And Sophia takes five grams and
dissolves it in 100 milliliters. We need to determine which student
made the most concentrated alkali solution.
Concentration is the amount of a
substance dissolved in a specified amount of water. If we have two solutions with the
same volume but different amounts of substance dissolved in them, we can easily
deduce which solution is more concentrated. The one with more dissolved
substance per unit volume is more concentrated. But in this question, the volumes
of the three solutions are different. So what do we do?
One way to solve this problem is to
take the same volume of each solution and find what mass would be in that
volume. Then it will be easy to determine
which is the most concentrated solution.
Here are the three solutions with
the masses of sodium hydroxide added written at the top and the volumes of water at
the bottom. So let’s take the same volume of
each solution. Let’s look at only 100 milliliters
of each. Now, we need to calculate what mass
is in each of the 100-milliliter volumes.
For Sophia’s solution, there is
five grams of sodium hydroxide in 100 milliliters of water. So we can write five grams in 100
milliliters for Sophia’s solution.
In Chloe’s solution, we know there
are 50 grams in 400 milliliters. Since 100 milliliters is a quarter
of 400 milliliters, we are only considering a quarter of the given volume. This means that we can find a
quarter of the mass. This will give us the mass in 100
milliliters. 50 grams divided by four is equal
to 12.5 grams. So there are 12.5 grams of sodium
hydroxide in 100 milliliters of Chloe’s solution.
In Daniel’s solution, we know there
are 20 grams of sodium hydroxide in 200 milliliters. If we consider only 100 milliliters
of Daniel’s solution, we need to consider half of the 200 milliliters. So we must therefore only consider
half of the mass given to us. We must divide 20 grams by two,
which gives us 10 grams of sodium hydroxide in 100 milliliters of Daniel’s
solution.
Now we can compare the
concentrations since we have the same volumes. Sophia’s solution has only five
grams in 100 milliliters. Then Daniel’s solution has 10 grams
for every 100 milliliters. And Chloe’s solution has 12.5 grams
in every 100 milliliters. So Chloe’s solution has the largest
mass per unit volume. So which student has the most
concentrated alkali solution? The answer is Chloe.
