Learning Goal:

To understand how buffers use reserves of conjugate acid and conjugate base to counteract the effects of acid or base addition on pH.

A buffer is a mixture of a conjugate acid-base pair. In other words, it is a solution that contains a weak acid and its conjugate base, or a weak base and its conjugate acid. For example, an acetic acid buffer consists of acetic acid, CH₃COOH, and its conjugate base, the acetate ion CH₃COO^-. Because ions cannot simply be added to a solution, the conjugate base is added in a salt form (e.g., sodium acetate NaCH₃COO).

Buffers work because the conjugate acid-base pair work together to neutralize the addition of H⁺ or OH^- ions. Thus, for example, if H⁺ ions are added to the acetate buffer described above, they will be largely removed from solution by the reaction of H⁺ with the conjugate base:

H⁺+CH₃COO^--->CH₃COOH

Similarly, any added OH? ions will be neutralized by a reaction with the conjugate acid:

OH^-+CH₃COOH --> CH₃COO^-+H₂O

This buffer system is described by the Henderson-Hasselbalch equation

pH=pKa+log[conjugate base]/[conjugate acid]   

A beaker with 125mL of an acetic acid buffer with a pH of 5.000 is sitting on a benchtop. The total molarity of acid and conjugate base in this buffer is 0.100 M. A student adds 5.00mL of a 0.380M HCl solution to the beaker. How much will the pH change? The pKa of acetic acid is 4.760.

Express your answer numerically to two decimal places. Use a minus (-) sign if the pH has decreased.

A buffer solution is prepared by dissolving 1.50 g each of sodium benzoate, C6H5COONa, and benzoic acid, C6H5COOH (Ka = 6.45 × 10-5), in 150.0 mL of solution. The molar mass of benzoic acid is 122.12 g/mol and that of sodium benzoate is 144.10 g/mol.

a) What is the pH of this buffer solution?

b) Which buffer component must be added to change the pH to 4.00? What mass of that component is required?

c) What volume of 2.0 M NaOH or 2.0 M HCl must be added to the original buffer in part a to change the pH to 4.00?

The vapor pressure of solid benzene is 299 Pa at -30ºC and 3270 Pa at 0ºC. The vapor pressure of liquid benzene is 6170 Pa at 10ºC, and 15,800 Pa at 30ºC. (a) Calculate the triple point of benzene (temperature and pressure at which solid, liquid, and vapor all coexist). This is obtained by using the Clausius-Clapeyron equation first to determine ∆vapH for the solid and the liquid, and then to give equations for the curves defining the solid-vapor equilibrium and the liquidvapor equilibrium, and solving both the latter equations simultaneously. (b) Calculate the enthalpy of fusion at the triple point, ∆fusH = Hliq – Hsol.

If 91.0g of oxalic acid and 79.0g of methanol react to form 89.0 g of dimethyl oxalate, what is the percent yield?

When you titrate 100 mL of 0.10 M HF with 0.10 M KOH

a) When will this titration reach the equivalence point?

b) Calculate pH when you add 5 mL of KOH.

c) Calculate pH when you add 50 mL of KOH.

d) Calculate pH at the equivalence point.

e) Calculate pH when you add 101 mL of KOH.

(show each step)

Calculate the pH of a buffer solution after the addition of 20.0 mL of 0.200 M NaOH to 80.0 mL of 0.0500 M HC3H5O2 and 0.0250 M NaC3H5O2. (Ka = 1.4 x 10^-5)

Your family is in the business of processing iron from iron ore and after taking Chemistry, you want to help the family business with this knowledge.

Your family business just received a 4000 lb truck load of iron ore from a new customer. You were provided with following background information:

-The iron in the ore has been reduced to Fe²⁺ before redox titration.

-The oxidizing agent used is a standardized 0.086M potassium per manganate (KMnO₄).

-A solution of 1.00 g iron ore required 25.00 mL of 0.086M KMnO₄to reach end point.

-The yield of iron from the iron ore in the entire process is 80%.

What is the Theoretical and Actual Yield from the truck load?

1. A student attempts to identify an unknown compound by the method used in this experiment. She finds that when she heated a sample weighing 0.5015 g the mass went down appreciably, to 0.3432 g. When the product was converted to a chloride, the mass went up, to 0.3726 g.

a. Is the sample a carbonate? ____________ Give your reasoning.

b. What are the two compounds that might be in the unknown? ____________ and ____________

c. Write the chemical equation for the overall reaction that would occur when the original compound was converted to a chloride. If the compound is a hydrogen carbonate, use the sum of Reactions 1 and 2. If the sample is a carbonate, use Reaction 2.

Write the equation for a sodium salt and then for a potassium salt.

d. How many moles of the chloride salt would be produced from one mole of original compound? ____________

e. How many grams of the chloride salt would be produced from one molar mass of original compound? Molar masses: NaHCO3 ____________ g Na2CO3 ____________ g NaCl ____________ g KHCO3 ____________ g K2CO3 ____________ g KCl ____________ g If a sodium salt, ____________ g original compound → ____________ g chloride If a potassium salt, ____________ g original compound → ____________ g chloride

f. What is the theoretical value of Q, if she has an Na salt? ____________ if she has a K salt? ____________ g.

What was the observed value of Q? ____________

h. Which compound did she have as an unknown?

What mass of water (in grams) forms from the reaction of 2.22 L of hydrogen gas and 2.24 L of oxygen gas? Both gases are at 726 torr and 7.8 ^oC.

For cyclohexane, C6H12, the normal melting point is 6.47°C and the heat of fusion at this temperature is 31.3 J/g. Find the freezing point of a solution of 110 mg of pentane, C5H12, in 16.45 g of cyclohexane. Assume an ideally dilute solution and that only pure cyclohexane freezes out.

A sample of an ideal gas has a volume of 3.65 L at 13.00 °C and 1.80 atm. What is the volume of the gas at 24.00 °C and 0.990 atm?

Given the half-cells:

Ni/Ni2+ and Fe/Fe2+

a) Chose the anode and cathode. b)Calculate Eo and DG for this reaction. c) Calculate what the new cell potential (E) would be if the non equilibrium concentrations were [Ni2+] =.40 M and [Fe2+] = 0.30 M. Calculate the how long you would have to run the battery at 6 amps to make this battery die at zero volts.

Put the terms in the appropriate blanks.

What is the difference between a 10% (v/v) methanol (CH₃OH) solution and a 10% (m/m) methanol solution?

A 10% (v/v) methanol solution can be made by adding _________ of methanol to _________ of water, while a 10% (m/m) methanol solution can be made by adding _________ of methanol _________ of water.

a. 10 mL

b. 90 mL

c. 90 g

d. 100 g

e. 100 mL

f. 10 g

How do temperature and pressure affect the solubility of solids and gases in water?

An increase in temperature usually __________ the solubility of solids and ________ the solubility of gases in water. An increase in pressure of a gas above a liquid _________ the solubility of a gas within that solution and _________ the solubility of solids.

a. increases

b. decreases

c. has no effect on

Describe the formation of an aqueous LiBr solution, when solid LiBr dissolves in water.

The _________ at the surface of the solid are pulled into solution by the _________ water molecules, where the ________ process surrounds

1. Which chemical is formed upon the addition of HCl to methyl orange? Name (not IUPAC) and draw it.

2. Which chemical is formed upon the addition of NaOH to methyl orange? Name (not IUPAC) and draw it. Explain how you know.

3. Which chemical is formed upon the addition of HCl to phenolphthalein? Name (not IUPAC) and draw it.

4. Which chemical is formed upon the addition of NaOH to phenolphthalein? Name (not IUPAC) and draw it. Explain how you know.