± PhET Simulation - Concentration Solutions consist of a solute dissolved into a

Question

± PhET Simulation - Concentration

Solutions consist of a solute dissolved into a solvent, e.g., a salt dissolved in water. There is often a need to quantify how much solute is present per unit solution volume. Concentration is most commonly expressed in terms of molarity (as molar or M), which is the number of moles (mol) per liter (L) of solution. A one molar solution can be expressed as follows:

1 M = 1 mol1 L or 1 molL

Every solute is limited in how much of can be dissolved into solution at a given temperature and pressure. These limitations exist because of the nature of dissolution, which involves the interaction between solute and solvent molecules. The point at which no more solute can be dissolved into solution is called the “saturation point”, and the concentration at which it occurs is considered the compound’s solubility. If too much solute is added to a solution, the concentration must be diluted by adding more solvent, or the saturation point must be raised by appropriately adjusting the temperature and sometimes the pH. A solution can be considered “dilute” when the concentration is less than half of the saturation point for the conditions, but dilute is a qualitative description that has no fixed definition.

Click on the image to explore this simulation, which shows how concentration is affected by various processes. When you click the simulation link, you may be asked whether to run, open, or save the file. Choose to run or open it.

When the simulation is opened, you will see a container filled to 12 L with water. You can select a solute using the Solute dropdown menu, and add it as a solid or concentration solution (using the respective radio buttons). The Concentration meter can be dragged (both the display and the sensor) to anywhere in the simulation, but the sensor must be submerged in the mixture to read the concentration. You can also add water and remove the mixture using the respective taps and evaporate the mixture using the Evaporation scale bar.

Part A

Use the PhET simulation to identify what happens to the concentrations of a dilute Drink mix mixture or a pure Drink mix solution for the following scenarios. A pure Drink mix solution can be obtained by draining all the mixture, then adding Drink mix to the container as a Solution (radio button in the upper right area of the simulation). Note that the Concentration meter can be placed toward the bottom of the container or in the stream of the Drink mix.

Drag the appropriate items to their respective bins.

Help

Reset

Add Drink mix solution to a diluted mixture of Drink mix in pure water

Add Drink mix solid to a diluted mixture of Drink mix in pure water

Evaporate water from a diluted mixture of Drink mix in pure water

Evaporate water from pure Drink mix solution

Drain the pure Drink mix solution

Drain the diluted mixture of Drink mix in pure water

Add water to pure Drink mix solution

Add Drink mix solid to pure Drink mix solution

Add water to a diluted mixture of Drink mix in pure water

Add pure Drink mix solution to pure Drink mix solution

Increases concentration

Decreases concentration

Does not affect concentration

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Calculating molarity

The concentration of a substance in solution can be calculated by dividing moles of the substance by the volume of solution. However, the number of moles cannot be directly measured. The mass can be directly measured using a balance, and the molecular weight or molar mass (MW) can be used to convert between mass and moles of a substance. The following equations describe the relationships between mole, volume, and molarity as well as mass and moles.

Mn==n/Vm/MW

where M represents molarity, n is the number of moles, V is the volume (in L), m is the mass (in g), and MW is the molar mass (in g/mol). These equations can be rearranged to solve for any unknown terms as long as the other values are known.

Part B

A chemist prepares a solution by adding 418 mg of Co(NO3)2 (MW = 182.94 g/mol ) to a volumetric flask, and then adding water until the total volume of the contents of the flask reaches the calibration line that indicates 500 mL . Determine the molarity of the prepared solution.

Express the concentration in moles per liter to three significant figures.

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Concentration of ions in solution and rearranging the molarity equation

Molarity often refers to molecular molarity. When salts dissolve in solution, each ion can be expressed as its own concentration. For example, when examining the dissolution of 1 M NaCl, the solution is 1 M in Na+ and 1 M in Cl. However, the dissolution of 1 M CoCl2 yields 1 M in Co2+ and 2 M in Cl because there are two chloride ions in each CoCl2 formula unit.

When the molarity of a solution is known the molarity equation can be arranged to solve for either the amount of moles of solute present in a given volume or the amount of volume needed to give a certain amount of moles.

moles of soluteliters of solution==liters of solution×molaritymoles of solutemolarity

Part C

Determine the mass of chloride (MW = 35.45 g/mol ) in grams present in 100 mL of a 0.255 M solution of aqueous GaCl3 (gallium(III) chloride).

Express the mass in grams to three significant figures.

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Dilutions

Solution concentrations can be easily reduced by adding additional solvent. This can be observed in the PhET simulation by adding water to any mixture while the Concentration meter is measuring solution concentration. Empty the container, and add Copper sulfate solution (with the dropper) until the container is filled to 12 L. The Concentration meter will read 1.000 mol/L. Then, fill the container with water from the tap, and the dilute concentration should read 0.500 mol/L (± 0.005 mol/L depending on how close to 12 L of copper sulfate solution you added).

When a solution is diluted, the number of moles of solute does not change, thus the new concentration can be easily calculated when diluting with a known volume, or the required new volume can be calculated using the desired final concentration. The following equation relates the initial concentration and volume to the final concentration and volume:

MconcVconc = MdilVdil

where M represents concentration and V represents volume for the concentrated (conc) and diluted (dil) solutions.

Part D

A beaker contains 428 mL of a 6.95 M HCl(aq) solution. Determine the new concentration of the solution after it is diluted by adding 113 mL of water.

Express the concentration in molarity to three significant figures.

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Help

Reset

Add Drink mix solution to a diluted mixture of Drink mix in pure water

Add Drink mix solid to a diluted mixture of Drink mix in pure water

Evaporate water from a diluted mixture of Drink mix in pure water

Evaporate water from pure Drink mix solution

Drain the pure Drink mix solution

Drain the diluted mixture of Drink mix in pure water

Add water to pure Drink mix solution

Add Drink mix solid to pure Drink mix solution

Add water to a diluted mixture of Drink mix in pure water

Add pure Drink mix solution to pure Drink mix solution

Increases concentration

Decreases concentration

Does not affect concentration

1 L Evaporation Solute: Drink mix Solid Solution Concentration (mol/L) Remove Solute

Explanation / Answer

Part B)

molarity = moles/L of solution

              = 0.418 g/182.94 g/mol x 0.5 L

              = 0.00457 M

Part C) 1 mole of GaCl3 has 3 moles of Cl

moles of Cl = 0.255 M x 0.1 L = 0.0255 mol

mass of Cl = 0.0255 x 35.45 = 0.904 g

Part D)

new concentration = old concentration x old volume/new total volume

                              = 6.95 M x 428 ml/(428 + 113) ml

                              = 5.498 M

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