Part 2 of Solution Chemistry Background Lab What

Part 2 of Solution Chemistry Background Lab

What is meant by “molarity? ” Let’s look at some examples. To illustrate, we are going to use a concentration unit of fish in water called “fishar”. Fishar = # of fish volume (L)

Concentration = V=1 L n = 2 fish # of fish volume (L)

Concentration = V=1 L n = 2 fish Concentration = 2 “fishar” 2 fish 1 L = 2 “fishar”

Concentration = 4 fish 1 L V=1 L n = 2 fish n = 4 fish Concentration = 2 “fishar” [ ] = 4 “fishar”

Concentration = 20 fish 5 L V=1 L V=5 L n = 2 fish n = 4 fish n = 20 fish Concentration = 2 “fishar” [ ] = 4 “fishar”

Now let’s do the same thing with moles. We are going to use a concentration unit of moles in water called “molar”.

Concentration = V=1 L n = 2 moles Concentration = 2 molar 2 moles 1 L

Concentration = 4 moles 1 L V=1 L n = 2 moles n = 4 moles Concentration = 2 molar [ ] = 4 molar

Concentration = 20 moles 5 L V=1 L V=5 L n = 2 moles n = 4 moles n = 20 moles [ ] = 4 molar Concentration = 2 molar

…from stock solutions

Concentration…a measure of solute-to-solvent ratio concentrated “lots of solute” vs. dilute “not much solute” “watery” In order to dilute a concentrated solution, you add more water.

concentrated stock solution

remove small sample of stock solution moles of solute concentrated stock solution

remove small sample of stock solution moles of solute concentrated stock solution add water and mix

remove small sample of stock solution moles of solute concentrated stock solution add water and mix same number of moles of solute in a larger volume diluted solution

remove small sample of stock solution moles of solute concentrated stock solution How do we calculate the number of moles of solute removed from the stock solution?

remove small sample of stock solution moles of solute concentrated stock solution

remove small sample of stock solution moles of solute concentrated stock solution moles of solute = molarity of stock solution x volume of stock solution moles = M x V

Stock solution: 1. 0 L of 4. 0 M Na. Cl Ex: How many moles of Na. Cl would be contained in 0. 50 L of a 4. 0 M Na. Cl stock solution?

Stock solution: 0. 5 L of 4. 0 M Na. Cl Ex: How many moles of Na. Cl would be contained in 0. 50 L of a 4. 0 M Na. Cl stock solution? moles = M x V = 4. 0 M x 0. 50 L = 2. 0 moles Na. Cl

Practice problems 3 – 5 on your lab sheet now.

Stock solution: 0. 5 L of 4. 0 M Na. Cl In order to dilute the original solution, we can take the 0. 5 L of the 4. 0 M stock solution and place it in another beaker and add water to bring the total volume back to 1. 0 L

Stock solution: 0. 5 L of 4. 0 M Na. Cl Na. Cl In order to dilute the original solution, we can take the 0. 5 L of the 4. 0 M stock solution and place it in another beaker and add water to bring the total volume back to 1. 0 L

Now the 2 moles are dissolved in 1 L of solution rather than 0. 5 L of solution. They have been diluted. It’s the same number of moles, but in more water. We can calculate the new molarity using the molarity equation. M = mol/V M = 2 moles / 1. 0 L M=2 M Na. Cl

An important point in dilution is that the moles of solute you use from the concentrated solution is the same as the moles in the new dilute solution that you make. molesconcentrate = molesdilute Na. Cl

An important point in dilution is that the moles of solute you use from the concentrated solution is the same as the moles in the new dilute solution that you make. molesconcentrate = molesdilute Mc V c = M d V d Na. Cl

Let’s see how this equation works. Ex: You want to make 1. 5 L of 0. 25 M Na. Cl from a stock solution of Na. Cl that has a concentration of 2. 8 M. What volume of the stock solution do you need to use to make your dilute solution?

Let’s see how this equation works. Ex: You want to make 1. 5 L of 0. 25 M Na. Cl from a stock solution of Na. Cl that has a concentration of 2. 8 M. What volume of the stock solution do you need to use to make your dilute solution? Givens: Mc = 2. 8 M Md = 0. 25 M Vd = 1. 5 L

Let’s see how this equation works. Ex: You want to make 1. 5 L of 0. 25 M Na. Cl from a stock solution of Na. Cl that has a concentration of 2. 8 M. What volume of the stock solution do you need to use to make your dilute solution? Givens: Mc = 2. 8 M Md = 0. 25 M Vd = 1. 5 L Unknown: Vc = ?

Let’s see how this equation works. Ex: You want to make 1. 5 L of 0. 25 M Na. Cl from a stock solution of Na. Cl that has a concentration of 2. 8 M. What volume of the stock solution do you need to use to make your dilute solution? Givens: Mc = 2. 8 M Md = 0. 25 M Vd = 1. 5 L Unknown: Vc = ? Equation: Mc. Vc = Md. Vd

Let’s see how this equation works. Ex: You want to make 1. 5 L of 0. 25 M Na. Cl from a stock solution of Na. Cl that has a concentration of 2. 8 M. What volume of the stock solution do you need to use to make your dilute solution? Givens: Mc = 2. 8 M Md = 0. 25 M Vd = 1. 5 L Unknown: Vc = ? Equation: Mc. Vc = Md. Vd Substitution: (2. 8 M) x Vc = (0. 25 M)(1. 5 L)

Let’s see how this equation works. Ex: You want to make 1. 5 L of 0. 25 M Na. Cl from a stock solution of Na. Cl that has a concentration of 2. 8 M. What volume of the stock solution do you need to use to make your dilute solution? Givens: Mc = 2. 8 M Md = 0. 25 M Vd = 1. 5 L Unknown: Vc = ? Equation: Mc. Vc = Md. Vd Substitution: (2. 8 M) x Vc = (0. 25 M)(1. 5 L) Solve: Vc = 0. 13 L = 130 m. L

Let’s see how this equation works. Ex: You want to make 1. 5 L of 0. 25 M Na. Cl from a stock solution of Na. Cl that has a concentration of 2. 8 M. What volume of the stock solution do you need to use to make your dilute solution? Givens: Mc = 2. 8 M Md = 0. 25 M Vd = 1. 5 L Unknown: Vc = ? Equation: Mc. Vc = Md. Vd Substitution: (2. 8 M) x Vc = (0. 25 M)(1. 5 L) Solve: Vc = 0. 13 L = 130 m. L What this means is that to make your dilute solution, you will need to measure out 130 m. L of the stock solution, add it to a 1. 5 L volumetric flask, and then add water to the line to dilute it.

Now practice some dilution problems on your own. Do problems 6 – 7 on your lab sheet.

Why do we use the volumetric flask instead of a beaker or graduated cylinder to make solutions? Device Volume Tolerance Possible volume range of measurement Cost Notes 1000 m. L beaker 1000 m. L graduated cylinder 1000 m. L volumetric flask

Why do we use the volumetric flask instead of a beaker or graduated cylinder to make solutions? Device 1000 m. L beaker 1000 m. L graduated cylinder Volume 1000 m. L Tolerance Possible volume range of measurement Cost Notes 1000 m. L volumetric flask 1000 m. L

Why do we use the volumetric flask instead of a beaker or graduated cylinder to make solutions? Device 1000 m. L beaker 1000 m. L graduated cylinder Volume 1000 m. L Tolerance ± 5% ± 6 m. L ± 0. 3 m. L Possible volume range of measurement Cost Notes 1000 m. L volumetric flask

Why do we use the volumetric flask instead of a beaker or graduated cylinder to make solutions? Device 1000 m. L beaker 1000 m. L graduated cylinder Volume 1000 m. L Tolerance ± 5% ± 6 m. L ± 0. 3 m. L Possible volume range of measurement Cost Notes 950 m. L to 1050 ml 994 m. L to 1006 m. L 1000 m. L volumetric flask 999. 7 m. L to 1000. 3 m. L

Why do we use the volumetric flask instead of a beaker or graduated cylinder to make solutions? Device 1000 m. L beaker 1000 m. L graduated cylinder Volume 1000 m. L Tolerance ± 5% ± 6 m. L ± 0. 3 m. L Possible volume range of measurement Cost Notes 950 m. L to 1050 ml 994 m. L to 1006 m. L $8. 40 $80. 15 1000 m. L volumetric flask 999. 7 m. L to 1000. 3 m. L $38. 25

Why do we use the volumetric flask instead of a beaker or graduated cylinder to make solutions? Device 1000 m. L beaker 1000 m. L graduated cylinder Volume 1000 m. L Tolerance ± 5% ± 6 m. L ± 0. 3 m. L Possible volume range of measurement 950 m. L to 1050 ml 994 m. L to 1006 m. L 1000 m. L volumetric flask 999. 7 m. L to 1000. 3 m. L Cost $8. 40 $80. 15 $38. 25 Notes Can be used for many different volumes Can only be used for one specific volume

Device 1000 m. L beaker 1000 m. L graduated cylinder Volume 1000 m. L Tolerance ± 5% ± 6 m. L ± 0. 3 m. L Possible volume range of measurement 950 m. L to 1050 ml 994 m. L to 1006 m. L 1000 m. L volumetric flask 999. 7 m. L to 1000. 3 m. L Cost $8. 40 $80. 15 $38. 25 Notes Can be used for many different volumes Can only be used for one specific volume Perform questions 8 through 10 now.