Question

Question 2 Salicylic acid, HC,H,0, is a chemical compound used in many facial cleansing products. Figure 2. Structure of Salicylic Acid a) Identify the ionizable hydrogen (HA or Hg) with the correct pk НА lonizable Hydrogen pK, 2.97 HA 13.7 :91 3 19- 10 b) The solubility of salicylic acid in water is 0.210-96 HC,HsOs by mass. What is the pH of an aqueous solution of salicylic acid at saturation? (Assume the density of the solution is 1.00 g/mL). 2-U 111 BU705 pH of salicylic acid solution c) A certain dermatological facial cream (16.0fl. oz., density-0.960 g/mL) contains C,Ho, by mass. How many grams of HC,H,0, are contained in the facial cream? ルohu x 24 s.het-y 3-11m Pa 452·248a b grams of HCMO, d) Construct a heat curve diagram for salicylic acid on the graph below. Include labels (and units) for the x-axis, y-axis, melting point, boiling point, dHfus. and Hvar Heat Curve Diagram for Salicylic Acid from 100. °c to 300. C Table 1. Physical Properties for Salicylic Acid 159 ℃ M.P 211 °C В.Р. 24.5 kJ/mol AWs 79.4 kJ/mol eo e) How much heat (in kJ) would be required to transition a pure sample of salicylic acid from solid → liquid (at the melting point) containing the same mass from Question 2c? 2 4. 21. S s4.4

Answer 1

2. (a) In structure given above in part (a) the H_A is attached to single O and when added to water it ionises as OH_A^- which is the cause of basicity of solution and we know for bases the pH ranges between (8-14) and also pKa = pH which means that value of pKa = 13.7 which corresponds to ionisable H_A.

Where as the H_B is attached to COO when added to water dissociates as H⁺ which is the cause of acidity in solution since acidic pH ranges between 1-6 therefore pKa = 2.97 corresponds to isonisable H_B

(b) Mass of Salicylic acid is 138 g and density of solution is given as 1 g/ml

volume of solution = density x mass = 1 x 138 ml = 138 ml

mass of soluble Salicylic acid in solution = 138 x (0.21%) = 0.2898 g

since the amount of H⁺ (hydronium ion) ion produced in solution is equal to amount of Salicylic acid soluble in water, because acid when dissolved in water behaves acidic i.e it dissociates as H⁺ ion.

Therefore concentration of [H⁺] = 0.2898/138 = 2.1 x 10^-3 g/ml

and we know pH = - log₁₀[H⁺] = -log₁₀(2.1 x 10^-3) = 2.677

(c) 1 fl oz = 29.57 ml

Therefore 16 fl oz = 16 x 29.57 ml = 473.12 ml

Mass of facial cream = density of facial cream x volume of cream = (0.960 g/ml) x 473.12 ml = 454.19 g

Given, that Salicylic acid in cream is 6% by mass

This means,

Mass of Salicylic acid = 6% of Mass of facial cream = 0.06 x 454.19 = 27.25 g

(d) Heat curve for Salicyclic acid is shown below,

300 °C Vaporization 211 °C --。 Heat of | Vaporization = Melting 159 °C--- Heat of fusion = m&Hfusi 100 °C Heatadded

(e) Mass of Salicylic acid = 27.25 g

Heat required for transition,

$Q =m\Delta H_{fus}$

Q = (27.25/137) mol x 79.4 kJ/mol = 15.67 kJ