# Titration Example Problems

Titration Example Problem Suppose that 10.0g of an unknown monoprotic weak acid, HA, is dissolved in 100 mL of water. To reach the equivalence point, 100.0mL of 0.10 M NaOH was used. After the addition of 50.0 mL, the pH of the solution was found to be 4.00. a) How many moles of base was added? b) State how many moles of acid were added initially? c) what is the molar mass of the acid? d) what is the value of pKa for the acid? e) what is the pH after 120.0 mL base was added. (and of course you should be able to answer, what is the pH at any point on the curve, do this at several points for practice) f) Would the volume to reach equivalence change if the initial volume the 10.0 g HA was dissolved in changed? (and of course you should be able to answer, what is the pH at any point on the curve, do this at several points for practice) Titration Example Problem Suppose that 10.0g of an unknown monoprotic weak acid, HA, is dissolved in 100 mL of water. To reach the equivalence point, 100.0mL of 0.10 M NaOH was used. After the addition of 50.0 mL, the pH of the solution was found to be 4.00. a) How many moles of base was added? b) State how many moles of acid were added initially? @ eq. Moles base added=moles acid initially=0.010 mol c) what is the molar mass of the acid?

MM = = d) what is the value of pKa for the acid? e) what is the pH after 120.0 mL base was added. (and of course you should be able to answer, what is the pH at any point on the curve, do this at several points for practice) f) Would the volume to reach equivalence change if the initial volume the 10.0 g HA was dissolved in changed? Titration Example Problem Suppose that 10.0g of an unknown monoprotic weak acid, HA, is dissolved in 100 mL of water. To reach the equivalence point, 100.0mL of 0.10 M NaOH was used. After the addition of 50.0 mL, the pH of the solution was found to be 4.00. a) How many moles of base was added? b) how many moles of acid were added initially? @ eq. Moles base added=moles acid initially=0.010 mol c) what is the molar mass of the acid? MM = = d) what is the value of pKa for the acid? pH at half equivalence= 4.00=pka e) what is the pH after 120.0 mL base was added. (and of course you should be able to answer, what is the pH at any point on the curve, do this at several points for practice) Excess OH- = 20mL

) f) Would the volume to reach equivalence change if the initial volume the 10.0 g HA was dissolved in changed? NO! Molar SolubilityThe amount of a compound that is soluble. Mols/L Calculated from Ksp Common Ion Effect and Solubility and Kf Just like in Keq, if you add it to a salt solution with a common ion it changes the solubility Lowered Solubility Fill in concentration to Ksp equation You may make an ice chart if it helps, but it is not necessary.

Increased Solubility: Kf If a solutions ion reacts with an ion from the insoluble compound the reactions can be combined by adding reactions, and multiplying the two K together. Le Chatliers Reminder: AB ( s ) + ()+ ( ) Ksp AgCl=5x10-1 Common Ion vs. Increasing solubility. Le Chateliers Principle. ow would you decrease the solubility of AgCl?

AgCl Add a common ion: either Ag+ or Cl- How many grams of AgCl dissolve in 0.5L of 0.1M NaCl? AgCl -x =(0.1+x)(x) 0.1x 0M +x 0.1M +x x 0.1+x

x = M (molar solubility) 5x10-12 *(0.5L)=3.58x10-10g AgCl Conceptual Topics: Common Ion vs. Increasing solubility. Le Chateliers Principle. How would you decrease the solubility of magnesium carbonate? How would you increase the solubility? 3 2+ + 3 Decrease: add a common ion Increase: add something that removes some product. What reacts with CO3-?

An Acid! Common Ion Effect and Solubility We can see if you mix two salt solutions if a precipitate will form. This is like using Q to determine which way the reaction will go. You can have two different salt solutions that are perfectly soluble, but when mixed form a precipitate See the picture for how this happens You can calculate if this will happen QK precipitate Ksp AgCl=5x10-13 A 10mL solution of 1.00x10-5 M silver nitrate and a 10mL solution of 1.00x10-5 sodium chloride are mixed. Does a precipitate form? =2.5x10-11

Q>K so there is a precipitate. A 10mL solution of 0.100 M silver nitrate and a 10mL solution of 0.1M sodium chloride are mixed. Does a precipitate form? Estimation: 0.1*0.1 >>than 10-13 Q>>K so there is a precipitate. A 10mL solution of 1.00x10-6 M silver nitrate and a 10mL solution of 1.00x10-6 sodium chloride are mixed. Does a precipitate form? =2.5x10-13 Q

Add H+ to balance Hs ( 2e*1=2)*3=6 8H++ 3 ZnS(s) +2 NO3-(aq) +2 -2 +5 -2 3Zn2+ (aq)+ 3S (s)+ 2NO (g) +4H2O +2 ( 3e*1=3)*2=6 0 +2 -2 Balance the following redox reaction in an acidic solution. Decide which is the oxidizing agent and which is the reducing agent. Describe the flow of electrons. Is the reaction spontaneous? Find G and K. 8H++ 3 ZnS(s) +2 NO3-(aq)

+2 -2 From Reduction Table: S/S2= -0.476 NO3-/NO = +0.96 +5 -2 3Zn2+ (aq)+ 3S (s)+ 2NO (g) +4H2O +2 0 +2 -2 E= 0.96 + 0.476 = 1.436 V 5 6 96485 1.436=8.31 1 0 1.436 6 ( )

0.0257 = = =335 0.0257 Cells Decide which is the oxidizing agent and which is the reducing agent for the following galvanic cell. Is the reaction spontaneous? What is the Ecell? What would happen to the mass of the anode vs the mass of the electrodes at the cathode. Describe the electron flow. Pb2+/Pb = -0.13V Au1+/Au = +1.69 V Higher Ered=cathode Galvanic=spontaneo us: so Yes spontaneous! Ecell= 1.69+0.13 =

1.82 V Au is reduced: making it the oxidizing agent Au electrode mass increases: Pb electrode mass decreases 1+

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