Pre-Lab Exercise: 1. What is a primary standard substance? Give an example. 2. W

Question

Pre-Lab Exercise: 1. What is a primary standard substance? Give an example. 2. What is meant by the term, equivalence point, during an acid-base titration? 3. What is an indicator? What role does it play in an acid-base titration? 4. Why is it necessary to standardize a NaOH solution? 5. Find the molar concentration of a NaOH solution if 0.3999 g of it is dissolved in water to yield 100.0 mL solution. Watch your sig. fig. 6. Consider the following equation KOH(aq) + HCl(aq) KCl(aq)+ H2O(l) Calculate the molar concentration of a KOH solution if 25.0 mL of it was required to completely neutralize 37.5 mL of a 0.0800 M HCl solution. 7. What is the criterion to decide whether you need to do more then three titrations?

Explanation / Answer

1)

primary standard are substances whose properties are well defined

example :

NaCl is used as a primary standard for silver nitrate reactions

2)

equivalence point is the point at which equivalent quantities of acid and base have been mixed.

3)

we know that

indicator is a weak acid or a weak base

The undissociated form of the indicator have a different color than the ionic form of the indicator

So

the indicator is used to find the equivalence point

4)

because NaOH is hygroscopic

so

it should be standardised

5)

we know that

moles = mass / molar mass

so

moles of NaOH = 0.3999 / 40 = 9.9975 x 10-3

now

molarity = moles x 1000 / volume (ml)

so

molarity = 9.9975 x 10-3 x 1000/ 100

molarity = 0.099975

so

the molar concentration of NaOH is 0.099975 M

6)

we know that

at equivalence point

Ma x Va = Mb x Vb

so

0.08 x 37.5 = Mb x 25

Mb = 0.12

so

the molar concentration of KOH is 0.12 M

7)

the volume of three titrations should be within 0.05 mL of each other

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