Analysis of a Mixture of Carbonate and Bicarbonate

Objective:   To become familiar with all aspects related to titrating, such as what titrations are, primary and secondary standards, its components, and the technique.

Equations:

HCl + NaOH à NaCl + H2O

HCO3 + H+  à H2CO3

CO3^2+ + 2H+  à H2CO3

Ba^2+ + CO3^2-  à BaCO3

HCO3 + OH à CO3^2- +H2O

Data:

# Unknown D

Measured weight unknown = 2.0426g

Measured weight NaOH= 2.0469g

 Flask NaOH (ml) NaOH needed to standardize HCl (ml) HCl needed to determine endpoint of unknown (ml) HCl needed to precipitate BaCO3 (ml) 1 25.3 32.2 87.5 49.2 2 24.7 31.4 85.9 49.6 3 24.0 30.8 86.0 49.8 Average 24.7 31.5 86.5 49.5

Calculations:

Preparation of .1M HCl:

(12M)(V1)=(.1)(1000)

=8.3ml

Preparation of .1M NaOH:

(.1 molNaOH/1L)*(40gNaOH/1molNaOH)*(500ml/1000ml)= 2g NaOH

Preparation of KHP needed:

(25ml)*(204.233g KHP/mol)*(.1mol KHP/1L KHP)*(1L/1000ml) = .5106g KHP

Molarity of NaOH:

.51g  KHP*(1mol KHP/204.233g KHP)*(1mol NaOH/1mol KHP) = .0025 mol NaOH

(.0025 mol NaOH/.02467L) = .101M NaOH

Molarity of HCl:

(.101mol NaOH/1L)*(1 mol HCl/1mol NaOH)*(.03147L NaOH/.025L HCl) =.127MHCl

Determination of relative percent carbonate and bicarbonate:

Mol OH = (.05L)*(.101mol NaOH/L)*(1mol OH/1mol NaOH) = .00505mol OH

Mol H+ = (.04953L)*(.127mol HCl/L)*(1mol H+/1mol HCl) = .00629mol H+

.00629-.00505 = .00124mol HCO2

(.127mol HCl/L)*(.03147LHCl)*(1mol HCO3/1molHCl)*(1/.025Lunknown) = .1598M HCO3

(.127mol HCl/L)*(.03147L HCl)(1 mol CO3^2-/2mol HCl)*(1/.025L unknown) = .0799M CO3^2-

Total Alkalinity:

[HCO3-] + 2[CO3^2-] = .1598 + 2[.0799] = .3196M

%HCO3 = (.00124 mol HCO3-)*(61gHCO3-/1 mol HCO3-)*(1/2.0426g unknown) *100 = 3.7%HCO3-

%CO3^2- = (.03196-.01598)*(60g CO3^2-/mol)*(1/2.0426g unknown)*100 = 46.9%CO3^2-

Conclusion:

This lab has made me familiar with titration techniques, the concept of standards and to be able to determine analytes present in a sample.  Errors could have occurred while titrating and with improper reading of the buret.  Weighing of solids could have also been in error due to estimations of weight according to the balances, and the transfer of solids.

After Lab Questions:

1. Primary standard – what all calculations are based on
2. Secondary standard – used as a control in indirect titrations
3. Indirect titration – done by adding known concentration to and unknown and indirectly find out how much unknown is present
4. Titrant – liquid used to titrate the solution being tested