# Question Video: Calculating the Most Concentrated Sodium Hydroxide Solution from Mass and Volume Values for Three Different Solutions Science

During an experiment in a laboratory, Daniel carefully weighs out 20 g of an alkali called sodium hydroxide and dissolves it in 200 mL of warm water. Chloe takes 50 g of sodium hydroxide from the same jar and dissolves it in 400 mL of warm water. Finally, Sophia takes 5 g of sodium hydroxide from the same jar and dissolves it in 100 mL of warm water. Which student has made the most concentrated alkali solution?

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### 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.