Prelab Question tl: Approximately how much NaOH do you need to carry out 5 ritra
ID: 1030225 • Letter: P
Question
Prelab Question tl: Approximately how much NaOH do you need to carry out 5 ritrations and 3 standardizations? Hit: how many mL of NaOH do you expect to use for each titration?) Don' forget that you need to rinse the buret thoroughly using the NaOH solution before filling it. Show all work. Is it a good idea to restandardize your NaOH? Why or why not? # Preparation o a Column o a Strong-Acid Cation Exchange Resin in its Hydrogen Form The volume of the resin in the glass tube shown in Figure 1 is 50 mL. The exchange capacity of a strong-acid cation exchange resin of the polystyrene-sulfonic acid type is 2.0 millimoles per milliliter of wet resin. The total exchange capacity of the column shown is thus-100 millimoles for a monovalent cation and-50 millimoles for a divalent (+2 charge) cation. Roughly 70 to 80% of capacity can be used effectively. The remaining capacity is left unused to insure completeness of the exchange reaction. Record should be kept of the use made of the resin column. Regeneration, i.e., returning the resin to its acidic form, is accomplished by passing 3 M hydrochloric acid through the column. Prelab uestion #2: How many monovalent samples (+1 charge) can be run through the column before it loses its ectiveness? (hint: asume that the resin volume is 50 mL, that 70% of the resin can be used effectively, that each mL of resin has 2.0 millimoles of exchangeable protons and that each sample run requires 45 ml of 0.1 M NaOH to reach the endpoint.) Show all work To construct a column, put about 50 mL of wetted resin into a beaker (contains approximately 75 g of a strong-acid cation exchange resin such as Amberlite IR-120 or Dowex 50X8). Above the stopcock of a glass column such as that shown in Figure i, place a plug of glass wool that is 1em in length. Transfer the wetted resin to the column, allowing the resin to fall gently through the water to form an even bed. Wash with water, drain off the water as necessary, and stir or shake gently to insure the removal of all air bubbles. In this and subsequent operations, do not allow the level of the liquid to drop below the top of the resin bed. # To charge the column, drain the excess water from the column, stopping when the liquid reaches the top of the resin bed. Prepare 300 mL of 3 M hydrochloric acid (concentrated hydrochloric acid is 12.1 M). Pass 150 mL of 3 M hydrochloric acid through the column adjusting the flow rate to 10 to 15 mL per minute. Wash with 100 mL of water (flow rate may be increased for water washes). Pass a second 150-mL of 3 M hydrochloric acid through the column at 10 to 15 mL per minute. Wash with 300 mL of deionized water increase flow rate), adding the water in portions and allowing the level of the liquid to # fall almost to the top of the packing before the next addition. Continue the washing with deionized water until one drop of 0.1 M sodium hydroxide added to 25 mL of eluate ollected in a graduated cylinder containing 5-6 drops of phenolphthalein turns pink Resin may be stored in a beaker with DI water in your lab cupboard. Place parafilm securely around beaker opening before storing. Prelab Question #3: What is the purpose O lushing the column with water after regeneration with HCI? What does the phenolphthalein indicator tell you in relation to this purpose Determination of the moles of the cation in a salt. Carry three samples through the following procedure. Weigh accurately 0.30 to 0.40 g of the dried salt into a 400 mL beaker. Add carefully 25 to 40 mL of deionized water and dissolve the salt by swirling gently. Transfer this solution quantitatively to the column of strong-acid cation exchange # 97Explanation / Answer
1. To calculate the amount of NaOH used for 5 titration and 3 standardization, we need to know the volume of acid that we are taking for titration. Suppose we are titrating a 10 ml strong acid with 0.1 NAOH than we need approx. 10 ml of NAOH(0.1 M) along with 2-3 ml to rinse the buret.
So, for 1 titration we need 13-14 ml NaOH(0.1 M)
For 5 titrations we need 53-54 ml NaOH
For each standardization 25ml(approx.) of NaOH(0.1 M) needed.
Therefore, the amount of NaOH needed to carryout 5 titration & 3 standardization is approx. 129 ml
We always need to re-standardized our NaOH before titration because NaOH solutions cannot be prepared by mass to be an exact concentration (solid NaOH is too reactive),therefore we standardized our NaOH solution by titrating it with weighed samples of a primary standard acidic substance, potassium hydrogen phthalate which has one acidic hydrogen atom, and reacts with NaOH on a 1:1 stoichiometric basis.