Question

5. 3. In Part 1 of the experiment (preparation of Ca²+ (aq) solution of known concentration), the volumes of water used in steps 1, 2, 3, and 5 are all approximate. Why doesn't it matter that these volumes are approximate rather than accurately measured? (Be specific.) 4. If a standard Ca? solution is 100 ppm and 50 mL of solution is used for the standardization, calculate the approximate volume of 2.5 mM EDTA+ solution needed to reach the end point of the titration. As you can see in the structure above, top of page 4), the indicator EBT has two dissociable protons (H') on hydroxide groups (-OH); they are weak acids and both of their pk, values are around -10. Likewise, EDTA has four dissociable protons and the associated values for pKare 2.0, 2.7.6.2, and 10.3. Which protons are more acidic - those on EBT or those on EDTA? This experiment will be conducted in buffer near pH = 10. Are the protons for EBT mostly dissociated or undissociated at this pH? What about the protons for EDTA? 6. Read the procedure carefully and prepare your laboratory notebook with appropriate tables for collecting the data and making observations.

Answer 1

Q3 In part 1 of the experiment approximate volumes of water is taken because it is complexometry titration which form 1:1 complex with metal ions. That is why small difference in volume of water does not make major difference.

Q4 100 ppm of CaCO3 means 1oo mg of CaCO3 dissolve in 1 liter of water.

So the Molarity of CaCO3 = weight of CaCO3 / (Molar mass of CaCO3)* volume (L)

= 100/100 *1 (Molar mass of CaCO3 = 100 g/Mol)

= 1 mM

Since Ca2+ and Mg2+ have same n factor

Hence according to Molarity Equation,

M1V1=M2V2

where M1= 1mM, M2= 2.5mM (Given) ,V1= 50mL, V2= ?

V2= M1V1/M2

= 1*50 / 2.5

= 20 mL

Q4 Since pKa value is the negative of Log Ka which means as the Ka increases pKa decreases and Ka value increases with increase in the concentration of H+ ion.

So least the pKa value more acidic nature shows

Due to having less pKa value of EDTA is more acidic than EBT.

Near the pH value 10 EBT will be in the form of HIn- so proton remain it self and EDTA will remain in the form of EDTA4-. which means all hydoxy protone will loose