# Chemistry Calculations and Concepts

Determination of the Molar Mass and Ionization Constant of a Weak Monoprotic Acid

Acetic Acid + Water yields Conjugate Base + Hydronium Ion Or HA + H20; H30+ + A-

Calculation of the ionization constant for acetic acid from the measured pH of acetic acid samples:

Acetic Acid titrated with Sodium Hydroxide and the following was determined or known before experiment:

Avg. molar concentration of acetic acid titrated = 0.0455 mol/L

Avg. pH = 3.19

Standardized NaOH = 0.097 mol/L

Mass concentration of acetic acid = 2.4 g/L

Concentration and Ka calculations:

1. Calculation of [H30+] = 10-3.19 = 6.46 X 10-4 ?? - (Is this the way to find the hydronium concentration?)

2. Calculation of [A-] =

3. Calculation of [HA] =

4. Calculation of Ka = [H30+] [A-] =

___________

[HA]

Measurement of the pKa of Acetic Acid by the Half-Neutralization Method

Using half the amount of NaOH required for equilibrium from part 1, the average pH vale was determined to be 4.63

1. Calculation of pKa =

2. Calculation of Ka =

3. Calculation of [H30+] =

4. Calculation of [A-] =

5. Calculation of [HA] =

Molar Mass and Ionization Constant of an Unknown Weak Monoprotic Acid

Unknown acid = 1.84 g/L

NaOH Titrant = 0.097 mol/L

Average pH = 2.60

Avg. molar mass = 0.04252 mol/L

1. Calculation of pKa =

2. Calculation of Ka =

3. Calculation of [H30+] =

4. Calculation of [A-] =

5. Calculation of [HA] =

Problem: OH- concentration of 0.050 M ammonia is 9.5 X 10-4

(a) Write the chemical equation for the dissociation of NH3 in H2O

NH3 + H2O; NH4 + OH-

(b) Equilibrium constant expression for reaction of ammonia with water

Kb = [OH-] [NH4+]

___________

[NH3]

(c) Calculate Kb using the above information

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#### Solution Summary

This solution demonstrates how to calculate many chemistry problems such as concentrations and writing chemical equations.

Chemistry Problems

1. The following reaction occurs:

CaCO3 + 2H- -> Ca2+ + CO2 +H2O

If a 25gm sample of calcium carbonate is added to a 75.5 ml HCl (density = 1.125gm/cm3, 26% HCl by mass), calculate the molarity of the HCL remaining in solution (if any) after the reaction is complete. Assume no volume change.

2. A sample of cocaine, C17H21O4N, is diluted with sugar C12H22O11. When a 1.00mg sample of this mixture is burned, 1.00ml of carbon dioxide (density 1.8gm/l) is formed. What is the percentage of cocaine in the mixture?

3. NH4ClO4 is used in a space shuttle along with aluminum metal. The products of the reaction are aluminum oxide, aluminum chloride, NO, and steam. The reaction mixture contains 5.75gm of Al and 7.32gm of NH4ClO4.

a. What is the theoretical yield of the Aluminum chloride? Note that not all of the chloride comes from NH4ClO4.

b. If 1.87gm of Aluminum chloride is formed, what is the percent yield?

4. A 1.58gm sample of C2H3X3 (g) has a volume of 297ml at 769mm of Hg and 35 degrees centigrade. What is element X?

5. What volume of ethane (C2H6), measured at 23.0 degrees centigrade and 752mm Hg, would be required to heat 855gm of water from 25 degrees centigrade to 98 degrees centigrade (H= -226.2kJ/mole)

6. Brass has a density of 8.25gm/cm3 and a specific heat of .363 J/gm degrees centigrade. A cube of brass 7.5mm on an edge is heated on a flame at temperature 95 degrees centigrade. It is then immersed into 20ml of water at 22 degrees centigrade in an insulated container. Assuming no heat loss, what is the final temperature of the water?

9. A water solution containing 28% by mass Iron(III) chloride has a density of 1.271 gm/cm3. Assuming ideal behavior calculate the:

a. Molarity

b. Molality

c. Osmotic pressure at 25 degrees centigrade

d. Boiling point

10. The solution of iron(II) hydroxide, Fe(OH) 2 is 1.43x10-3 gm/liter at 25 degrees centigrade.

a. Write a balanced equation for the solubility at equilibrium.

b. Write the expression for the solubility product constant Ksp, and calculate its value.

c. Calculate the pH of a saturated solution of Fe(OH) 2 at 25 degrees centigrade.

d. A 50ml sample of 3.00x10-3 molar FeSO4 solution is added to 50ml of 4.00x10-6 molar NaOH solution. Does a precipitate of Fe(OH) 2 form? Explain and show calculations to support your answer.