Aspirin in water, pH 2.87 by quadratic, then the ionized fraction at pH 1.50 and 7.40: that sequence is the SC190 Unit 4 acid base analysis described here. Searches like "sc 190 unit 4 assignment example", "sc190 unit 4 sample" and "sc190 unit 4 example" land here.
What a finished SC190 Unit 4 acid base analysis looks like
Three calculations and a physiological reading of each. The first sets a reference: 0.030 M hydrochloric acid, fully dissociated, has a pH of 1.52, a rough stand-in for gastric fluid. The second finds the aspirin concentration, 0.325 g over 180.16 g/mol in 0.2500 L, or 7.22 x 10^-3 M, and sets up Ka equals x squared over C minus x with Ka of 3.2 x 10^-4. The shortcut gives 20.9 percent ionization, far past the 5 percent threshold, and the analysis switches to the quadratic: x equals 1.36 x 10^-3 M, pH 2.87, 18.9 percent ionized. The third applies the Henderson-Hasselbalch relationship as chemistry: at pH 1.50 the ratio of ionized to neutral aspirin is 0.0100, about 1 percent ionized, and at 7.40 it is roughly 7.9 x 10^3, or 99.99 percent ionized.
How a SC190 Unit 4 example is structured
Definitions are stated once at the top, pH as the negative log of hydrogen ion concentration, Ka as the equilibrium constant for dissociation and pKa as its negative log. Strong and weak acids are separated at the start because they need different methods: the hydrochloric acid line takes one step, while the aspirin line needs an equilibrium. Each calculation lists its givens, shows the setup, solves and reports pH to two decimal places, matching the two significant figures in Ka. The ionization-fraction section rearranges Henderson-Hasselbalch into a ratio, evaluates it at each pH and converts the ratio into a percentage. The physiological reading comes last and is limited to what the chemistry supports: neutral molecules cross lipid membranes more readily than ions, so the fraction neutral in each compartment is the quantity that matters for absorption.
Strong acid as the baseline
Hydrochloric acid at 0.030 M gives pH 1.52 in one line, since it dissociates completely. That line sets a scale against which the weak acid's behavior can be read.
An approximation tested and rejected
The shortcut predicts 20.9 percent ionization, so the analysis rejects it on the 5 percent rule and solves the quadratic, moving the pH from 2.82 to 2.87.
Ionized fraction from pKa
Rearranged as a ratio, Henderson-Hasselbalch gives ionized over neutral as 10 raised to pH minus pKa, evaluated at 1.50 and at 7.40 with each step shown.
Two compartments, two dominant forms
Aspirin is about 99 percent neutral in the stomach and more than 99.9 percent ionized in blood, and the analysis states both as fractions before interpreting either.
Membranes read from the fractions
Neutral molecules pass lipid membranes more readily, so the analysis concludes that gastric conditions favor the form that can cross, while plasma holds aspirin mostly as the ion.
Where marks go in SC190 Unit 4
Analyses lose most when a weak acid is treated as strong, taking the full concentration as hydrogen ion concentration and reporting a pH near 2.14 for the aspirin solution. The shortcut applied without the 5 percent check is the next frequent loss, because prompts often choose concentrations where it fails. Henderson-Hasselbalch ratios inverted, neutral over ionized read as ionized over neutral, reverse the physiological conclusion and cost the interpretation marks along with the arithmetic. pH values reported to four decimal places from two-figure constants draw precision comments. Physiological statements that go beyond the chemistry, dosing claims or treatment advice, are marked off-task in most sections. The smaller deductions go to logarithms taken of concentrations with units attached and pKa values quoted without a source.
Get a SC190 Unit 4 example written to your instructions
Paste in the Unit 4 question, its concentrations and constants and the physiological setting described, with the SC190 rubric attached. Each pH is computed with the right method, every approximation tested, and the fractions read against that setting without drifting into dosing. Delivery is typically 24-48h, and the first one costs you nothing.
SC190 Unit 4 questions, answered
Why does the approximation fail for aspirin?
Because aspirin is a fairly strong weak acid at a low concentration. With Ka of about 3.2 x 10^-4 and only 7.22 x 10^-3 M present, nearly a fifth of the molecules dissociate, so subtracting x from the starting concentration is not negligible. The 5 percent test catches this, and the quadratic gives the correct pH.
Is Henderson-Hasselbalch only for buffers?
No. It is a rearranged form of the Ka expression and holds for any weak acid at equilibrium with its conjugate base. Here it gives the ratio of ionized to neutral aspirin at a set pH, which is how drug ionization is usually calculated. Buffer design, often the next unit, uses the same equation for a different purpose.
How much physiology should the analysis include?
Enough to interpret the numbers and no more. The prompt usually wants the calculated value connected to one consequence, such as which form dominates where. Statements about dosing, side effects or treatment go beyond an acid base analysis in a chemistry course, and graders tend to mark them as outside the question rather than as extra credit.