SOLUTIONS

A solution is a homogeneous mixture of two or more substances; the dissolved substance is called the solute and the substance in which the solute is dissolved is called the solvent. The relative amounts of the substances in the solution determine which is solvent and solute; the one in the greater amount is solvent and solute; the one in the greater amount is considered the solvent.

 

PERCENT COMPOSITION - Think in terms of 100. Either grams per 100 grams or grams per 100 milliliters or milliliters per 100 grams or milliliters per 100 milliliters. You will be working with amounts other than 100 milliliters, so this is taken into account by the formula below

PERCENT = PART x 100
                    WHOLE

1. Percent - Mass in a total volume (m/v) Most frequently used form of %.
    A 5% solution means 5 grams in 100 mL volume. Or 2.5 grams in 50 mL volume.
    A 5 mg% solution means 5 mg in 100 mL volume. Same as 5 mg/dL. (dL = 100 mL)

2. Percent- Mass of solute and mass of solution (m/m)
    This refers to the amount of solute in grams in the total solution also in grams.

Example: A 10% aqueous NaCl solution contains 10 g of NaCl in 90 g of water (or 10 g NaCl per 100 g of solution).

3. Percent - Volume of solute in volume of solution. (v/v)
    This refers to relative amounts of two liquids in solution. Example: An alcoholic beverage that is 7% alcohol contains 7 ml of alcohol per 100 ml of solution.
    Ratios of individual liquids is generally given rather than %. Example: A chromatography solvent was chloroform: methanol: water (6:3:1).

Problems

1. How many grams of sugar are needed to make 100 ml of a 20% solution?

 

2. What is the concentration in mg % of 100 mg of NaCl dissolved in 500 ml of solution?

 

3. How many grams of sugar are needed to make 150 ml of a 30% solution:

 

4. I have 250 ml of a 0.9% NaCl solution (saline). How many grams of salt do I have?

 

5. How many grams of sugar are need to make 50 grams of a 3% solution?

 

MOLAR COMPOSITIONS OR SOLUTIONS Think in terms of LITERS.

        Molar solutions are used more frequently and are more precise than the percent solutions.

Molarity = moles
                 liters

1. How would I make up 500 mL of a 1 M solution of NaCl?

        moles =  grams                         Molarity = moles
                        MW                                             L

                                                        1           =  x moles
                                                                           0.5 L

                                                         x          =  0.5 moles

        0.5    =    x grams
                         58.5 

          x     =    29.25 grams

                            Put 29.25 g NaCl in flask, stir, add H2O and bring up to 500 ml

2. How would we make up 250 mL of a 3 M solution of NaCl?

 

3. How much NaCl would I need to make up 10 ml of a 6 M solution?

 

4. How many grams of Ca(CO3) would I need for 100 ml of a 2 M solution?

 

5. I have 29.25 g NaCl in 500 ml. What is its molarity?

 

Normality and Molarity

Molarity is moles per liter of solution.

M = moles
         liter

We use Normality when discussing solutions especially acids and bases.

HCl + Na(OH) ----> NaCl + HOH
acid        base              salt       water

We need 1 mole of acid H+ to neutralize 1 mole of base (OH)-
In dealing with acids and bases, we can refer to a mole of acid as an equivalent of acid or a mole of base as an equivalent of base.

If I had 1 mole of HCl, I=d have 1 mole of H+ and 1 mole of Cl-

If I had 1 mole of H2(SO4), I=d have 2 moles of H+ and 1 mole of (SO4)-2 Why 2 H+?

1 mole of HCl has 1 EQUIVALENT OF H+

1 mole of H2(SO4) has 2 EQUIVALENTS OF H+

We use the term NORMALITY for acids and bases - the number of moles of H+ or (OH)-

1 M HCl = 1 N HCl
1 M H2(SO4) = 2 N H2(SO4)
1 M H3(PO4) = 3 N H3(PO4) Why 3?

1 M NaOH = ____ N NaOH
1 M Ca(OH)2 = _____N Ca(OH)2

The normality of a solution will always be greater or the same as the molarity of the solution. If the normality of a solution has a number less than the molarity, the answer is wrong.

 

Problems

1) 0.2 M H2(SO4) = ____ N H2(SO4)

Setup of problem

1 M H2(SO4) = 2 N H2(SO4)

0.2 M H2(SO4 = x N H2(SO4)

1 M     =     2 N
0.2 M          x N

Criss cross multiply x = 0.4

 

2) 0.3 N H3(PO4) = _____ H3(PO4)

Setup of problem

1 M H3(PO4) = 3 N H3(PO4)

x M H3(PO4) = 0.3 N H3(PO4)

 

3) 0.4 N Ca(OH)2) = _______Ca(OH)2)

NEUTRALIZATION

If I had 50 H+'s how many (OH)- s would I need to neutralize it?
If I had 50 moles H+, how many moles (OH)- would I need to neutralize it?
If I had 50 equivalents of H+, how many equivalents (OH)- would I need to neutralize it?

Equivalents = Normality x Liters             Molarity = moles
                                                                                   L

moles = Molarity
    L
moles = Molarity x L

Equivalents = Normality x L

Milliequivalents = Normality x Milliliters Meq = (mL)(N)

 

If I had 50 mL of a 2 N H(SO)4 how many milliequivalents of H+ do I have?

How many milliequivalents of hydroxyl would I need to neutralize it?

meq H+      =     meq (OH)-

V1 N1         =     V2N2

(mL) (N)    =     (mL) (N)

1. How many milliliters of a 0.02 N NaOH would be needed to neuralize 35milliliters of a 0.15 N HCl?

2. What is the normality of a solution of HCl, if 50 mL of the HCl were neutralized by 30 mL of a 0.15 N NaOH?

DILUTIONS

Suppose I had 12 N HCl and wanted to make up 200 mL of 0.1 N solution. How many milliliters of the concentrated solution would I need to make up 200 mL of the dilute solution?

How many milliequivalents of H+ would I need in 200 mL of 0.1 N HCl

Meq = (mL) (N)

 

So how many milliequivalents would I need to take out of the concentrated HCl solution?

How many milliliters would have that number of milliequivalents?

Simple formula for dilutions

meq = meq

V1N1 = V2N2

How many milliliters of a 6 N NaOH would I need to make up 100 mL of a 1 N NaOH?

Problems

1. How many milliliters of 2N HCl would be needed to make 100 mL of 0.1 N HCl?

2. What is the Normality of a NaOH solution if it took 25 mL of a 0.1 N HCl to neutralize 10 mL of the NaOH?

3. How many milliequivalents of H+ are there in 200 mL of 0.25 N solution?

4. What is the normality of a HCl solution if 10 mL of 12 N HCl were brought up to a 1 L volume?

5. How many milliequivalents of NaOH are needed to neutralize 50 mL of a 0.2 N HCl solution?

6. How many grams of NaOH are needed to make 100 mL of 0.5 N NaOH? (hint: N = M for NaOH moles = grams/ MW)

7. A solution contained 30 mL of water and 70 mL of alcohol. Which one is the solvent?

8. What is the percent concentration (m/m) of a solution that had 20 grams of NaCl (MW 58.5) in 100 grams of water?

9. What is the percent concentration (m/v) of a solution which had 15 grams of a solute in a total volume of 30 mL of solution?

10. How many milligrams are present in 50 mL of a 5 mg% solution?

11. What is the Molarity of a solution that had 5.85 grams of NaCl (MW 58.5) in 200 mL of solution?

12. What is the Molarity of a solution which had 4 moles of NaOH (MW 40) in 500 mL of solution?

13. How many grams of NaOH (MW 40) are present in 200 mL of a 2 M solution?

14. What volume of a 0.5 M solution of HCl (MW 36.5) would contain 3.65 grams of HCl?

15. How many grams of NaCl are present in 15 mL of a 2.0% solution?

16. How many grams of solute are present in 375 mL of 7.5 M CaCl2 solution?

17. Given a 2.0 M solution of NaCl. How many milliliters of the solution would be needed to get 1.0 gram of NaCl?

18. Concentrated HCl is 12 N. How many milliliters of the concentrated acid wuld be needed for 100mL of a 0.15 N solution?

19. 5.85 grams of NaCl wer dissolved in water. The final volume was 100 mL. A) What is the molarity? B) What is % w/v?

 

Wednesday, March 21, 2007 01:28:02 PM