Before trusting its recipe, SC190's Unit 5 buffer design brief rejects acetate, calculates 4.71 g and 8.63 g of phosphate salts, and survives a 0.0100 mol acid challenge. Searches like "sc 190 unit 5 assignment example", "sc190 unit 5 sample" and "sc190 unit 5 example" land here.
What a finished SC190 Unit 5 buffer design brief looks like
A four-page brief in memo form with a specification, a selection argument, a recipe and a verification. The specification states pH 7.40, total phosphate 0.100 M, volume 1.00 L. The selection section compares candidate pairs by pKa and rejects acetate because reaching 7.40 would need a base-to-acid ratio near 437 to 1, leaving almost no acid to neutralize added base. For phosphate, the ratio of hydrogen phosphate to dihydrogen phosphate is 10 raised to 0.19, or 1.55, which splits 0.100 mol into 0.0608 mol and 0.0392 mol. At molar masses of 141.96 and 119.98 g/mol, the recipe calls for 8.63 g of Na2HPO4 and 4.71 g of NaH2PO4. The verification adds 0.0100 mol of HCl and recalculates: pH falls only to 7.22, where the same acid in pure water would reach 2.00.
How a SC190 Unit 5 example is structured
The brief is written as a design document, so it opens with requirements rather than theory: target pH, concentration, volume and any constraint on reagents. Selection follows and is argued from one rule, a buffer works when the target lies within about one unit of the acid's pKa, applied to each candidate with its numbers. The recipe section works from the Henderson-Hasselbalch ratio to moles to masses in one chain, with anhydrous salts specified and molar masses cited. Verification is its own section, not an afterthought: a strong acid challenge and a strong base challenge are each computed by adjusting the moles of both partners and recalculating, and each result is compared with the unbuffered case. A limitations paragraph closes the brief, noting that ionic strength shifts the effective pKa, so the final pH is set with a calibrated meter.
Requirements before chemistry
Target pH, total concentration and volume open the brief as a short specification, so every later choice can be checked against a stated requirement.
Candidates screened by pKa
Acetate fails because 7.40 lies 2.64 units above its pKa; phosphate passes at 0.19 units. The brief states the one-unit rule and applies it with numbers.
Ratio to moles to grams
A ratio of 1.55 divides 0.100 mol of phosphate into 0.0608 and 0.0392 mol, which become 8.63 g and 4.71 g of the two sodium salts.
Challenged from both directions
Adding 0.0100 mol of acid moves the pH to 7.22, and the same amount of base moves it to 7.59, each change set beside the five units unbuffered water would shift.
The meter has the last word
Calculated pH ignores ionic strength, which lowers the working pKa of phosphate at this concentration, so the brief specifies adjusting the final pH with a calibrated meter.
Where marks go in SC190 Unit 5
The heaviest loss is a pH computed for a mixture never shown to buffer, since that check is what the unit is built to reward. Pairs chosen outside the one-unit window, or a strong acid paired with its conjugate, produce a recipe that calculates neatly and fails in use. Ratios inverted in the Henderson-Hasselbalch step put the larger amount on the wrong salt and land the design near pH 7.02 instead of 7.40. Masses computed from hydrated molar masses when anhydrous salts are specified, or the reverse, shift the recipe silently. Verification that adds acid but never base, or recalculates pH without adjusting the moles of both partners, leaves the capacity claim half tested. Presenting a calculated pH as exact, with no word about ionic strength, draws a comment in many rubrics.
Get a SC190 Unit 5 example written to your instructions
Give us the Unit 5 specification, target pH, volume, concentration and permitted reagents, and the grading criteria SC190 applies. The brief screens candidates by pKa, builds the recipe to masses and challenges it with acid and base before calling it a buffer. It is generally back within 24-48h, free on a first request.
SC190 Unit 5 questions, answered
Why does a buffer need to be within one pH unit of its pKa?
Because outside that window one partner dominates so heavily that little of the other is left to react. One unit away, the ratio is already 10 to 1; at 2.64 units, acetate at pH 7.40 would sit near 437 to 1. Added base would then meet almost no weak acid, and the pH would move sharply.
What is buffer capacity, and does the brief need to calculate it?
Capacity is how much strong acid or base a buffer absorbs before its pH changes significantly, and it rises with total concentration. Most SC190 prompts want one or two challenge calculations rather than a formal capacity value. Adding a stated amount of acid and of base and reporting each new pH usually satisfies that requirement.
Which buffer pair should the brief use?
Whichever the prompt specifies, and if the choice is open, the pair whose pKa sits closest to the target. Phosphate fits pH 7.40 well; acetate fits roughly 3.8 to 5.8; ammonia and ammonium fit around 9.25. The design method is identical for each, and the selection argument shows the choice was reasoned rather than copied.