Safety is always, an ongoing concern in the laboratory. One of the most common i

Question

Safety is always, an ongoing concern in the laboratory. One of the most common incidents in undergraduate labs arise when chemicals go unlabeled and are than used inappropriately as a result. Suppose you find a jar labeled HCl, but no concentration is included on the label. In order to determine the concentration of the HCl you titrate 30 ml of it with 0.200 M NaOH. a. Write the titration reaction including phase labels. b. Suppose the titration requires 25 ml of 0.200 M NaOH. What is the concentration of the unlabeled jar of HCl? suppose you go beyond the equivalence point and add a total of 40 mL of 0.200 M NaOH to Cl solution. What is the pH of this new solution?

Explanation / Answer

a) Reaction between HCl and NaOH is

HCl (aq) + NaOH (aq) -----> NaCl (aq) + H2O (l)

b)

GIven we have taken 30 ml of HCl

V1 = 30 ml

Molarity of HCl be M1

Given 20 ml of 0.2 M NaOH is required

Volume V2 = 20 ml

Molarity M2 = 0.2 M

V1 * M1 = V2 * M2

30 ml * M1 = 20 ml * 0.2 M

M1 = 0.133 M Molarity of HCl

c) given total of 40 ml of 0.2 NaOH is added

equivalence point is achieved at 20 ml of NaOH so

amount of excess NaOH remaining in solution = 40 - 20 = 20 ml = 0.02 L

No. of moles of NaOH remaining = 0.02 * 0.2 mol/L = 0.004 moles

NaOH (aq) ------> Na+ (aq) + OH- (aq)

No. of moles of OH- = No. of moles of NaOH = 0.004 moles = [OH-]

pOH = - log ([OH-]) = - log (0.004) = 2.4

pH = 14 - pOH = 14 -2.4 = 11.6 Answer

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