Determining Gram Equivalent Mass and Ka of a Weak Acid Acids are substances that

Question

Determining Gram Equivalent Mass and Ka of a Weak Acid Acids are substances that contain ionizable hydrogen atoms within the molecule. Strong acids ionize completely, weak acids partially. An acid, whether strong or weak, may contain one or more ionizable hydrogen atoms in the molecule. An equivalent is the number of hydrogen atoms furnished when one mole acid ionizes. For example, HCI contains one ionizable hydrogen atom. and 1 equivalent per mole. H2SO4 has two ionizable hydrogen atoms, 2 equivalents per mole. The gram equivalent mass of an acid is defined as the number of grams of acid to furnish one mole of HSo,the molar mass of HCl, 36.45 g/mole, is equal to the equivalent mass 36.45 g/equiv because there is 1 equivalent per mole. 36.45 g1mo36.45g/equiv mole equiv Sulfuric acid, H2504, 2 equivalents per mole, has a molar mass 98.07 g/mole but an equivalent mass, 49.04 g/equiv. [seomoleIG quiv)"9.04g/equiv 98.07 g1 The equivalent mass may be determined by titrating an acid with a standard solution of a base. Although molarity can be used to measure the concentration of acid/bases it is not a useful measurements for neutralization reactions. Why? Because not every acid/base can add/remove the same number of hydrogen/hydroxide ions from solution HCI + NaOH One mole HCI neutralizes one mole of NaOH H2so, + 2 NaOH Na2SO4+2H20 (2) One mole H2504 neutralizes two mole of NaOH NaCl + H2O e.g.

Explanation / Answer

b) Equivalence point Volume = 32 mL from graph and table.

Eq(5):

#eq = N x V

For NaOH, Normality = Molarity = 0.1 N

#eq = 0.1 x (32/1000) = 0.0032

c) #eq (base) = #eq(NaOH) = 0.0032

At the equivalent point,

#eq (acid) = #eq (base) = 0.0032

d) #eq (acid) = grams of acid / equivalent mass of acid

Equivalent mass of unknown acid = 0.653 / 0.0032 = 204.0625 g

e) Half Equivalence point,

#eq = 0.0032 / 2 = 0.0016

And pH = pKa,

From Table,

V = 16 mL

f) At V = 16 ml, pH = 5.4

pH = pKa = 5.4

Ka of unknown acid = 10-5.4 = 3.98 x 10-6

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