Quiz & test questions Grade 10 Math

Stonebridge Falls Chlorination Quiz: Dose, Demand, and pH

A grade 10 quiz mixing multiple-choice, short-answer, and computation items on chlorine dosing, chlorine demand, and pH for a fictional town's treatment plant.

Grade: Grade 10
Subject: Math
Material: Quiz & test questions
Time: 30 min
Prep: No prep
Materials: Calculator

Standards

HSN-Q.A.1 Common Core Math

Use units as a way to understand problems and to guide the solution of multi-step problems; choose and interpret units consistently in formulas; choose and interpret the scale and the origin in graphs and data displays.

How this material aligns: Items 2, 4, and 8 carry concentration in milligrams per liter and flow in million gallons per day through the pounds formula to a feed rate in pounds per day, and item 7 converts a percent solution to milligrams per liter, using units to guide each step.

All materials for HSN-Q.A.1 →

HSN-Q.A.3 Common Core Math

Choose a level of accuracy appropriate to limitations on measurement when reporting quantities.

How this material aligns: Item 3 asks students to report a chlorine demand at a level of accuracy matched to the residual measurement, the appropriate-precision reporting this standard describes for a lab result.

All materials for HSN-Q.A.3 →

Download Word file

This material is in the public domain (CC0) and may be copied, edited, and shared freely, without attribution. Updated 2026-07-23.

Objectives

Students will use the pounds formula to find a plant's chlorine feed rate.
Students will calculate chlorine demand from dose and residual.
Students will judge whether a residual and a pH reading fall within stated ranges.

Vocabulary

pH: pH is a number that tells whether water is acidic, like lemon juice, or basic, like baking soda mixed in water. Pure water sits in the middle of the scale at a pH of 7.

Also: chlorine dose, residual, chlorine demand, hypochlorite

Teacher overview

This 30-minute quiz is worth 30 points and checks whether students can apply chlorination math and water-quality benchmarks to a fictional treatment plant. It mixes two multiple-choice items, two short-answer items, and five computation items built around the town of Stonebridge Falls, a fictional town used for this assessment. Students work with the pounds formula, the chlorine-demand relationship, a percent-to-concentration conversion, and the drinking-water pH standard.

Stonebridge Falls Chlorination Quiz: Dose, Demand, and pH — Grade 10 Math

Name: ____________________________ Date: ______________

Assessment

The town of Stonebridge Falls operates a water treatment plant that treats 3.0 million gallons per day (MGD). Use the pounds formula, loading (pounds per day) = concentration (mg/L) × flow (MGD) × 8.34, and chlorine demand = dose − residual, where needed.

(2 points) The maximum residual disinfectant level (MRDL) for chlorine in drinking water is:
- A) 0.3 mg/L
- B) 2.0 mg/L
- C) 4.0 mg/L
- D) 10 mg/L
(4 points) The plant treats 3.0 MGD and applies a chlorine dose of 2.5 mg/L. Using the pounds formula, find the chlorine feed rate in pounds per day.
(4 points) After the contact tank, the measured chlorine residual is 0.9 mg/L. Using chlorine demand = dose − residual, find the chlorine demand for the 2.5 mg/L dose. Report your answer to the same tenth-of-a-milligram precision as the residual reading.
(4 points) Find the pounds per day of chlorine consumed by demand (the part not left as residual), using your demand from item 3 and the 3.0 MGD flow.
(3 points) The plant's finished water has a pH of 7.4. The secondary standard for drinking-water pH is 6.5 to 8.5. Is the finished water within the standard? Explain in one sentence.
(3 points) A chlorine residual of 0.9 mg/L measured out in the distribution system is best described as:
- A) below the typical 0.2 to 2.0 mg/L range utilities maintain
- B) within the typical 0.2 to 2.0 mg/L range utilities maintain
- C) above the 4.0 mg/L MRDL
- D) exactly at the MRDL
(4 points) The plant stores a 12 percent sodium hypochlorite solution. Using the conversion that a 1 percent solution equals 10,000 mg/L, find the concentration of the 12 percent solution in milligrams per liter.
(4 points) During a summer peak the plant raises its flow to 4.0 MGD while keeping the same 2.5 mg/L dose. Find the new chlorine feed rate in pounds per day.
(2 points) Explain why an operator measures the chlorine residual far out in the distribution system, not only in the finished water leaving the plant.

From Water for Schools (waterforschools.org). This material is in the public domain (CC0) and may be copied, edited, and shared freely.

Answer Key

Answer: C) 4.0 mg/L.
The maximum residual disinfectant level for chlorine is 4.0 milligrams per liter. Choice B is the top of the range utilities typically maintain, not the regulatory maximum, and choice A sits far below the maximum, so both are distractors.
Answer: 62.55 lb/day.
Loading = concentration × flow × 8.34 = 2.5 mg/L × 3.0 MGD × 8.34 = 7.5 × 8.34 = 62.55 pounds per day.
Answer: 1.6 mg/L.
Chlorine demand = dose − residual = 2.5 − 0.9 = 1.6 milligrams per liter, reported to the same tenth-place precision as the 0.9 residual reading.
Answer: 40.03 lb/day.
Demand load = 1.6 mg/L × 3.0 MGD × 8.34 = 4.8 × 8.34 = 40.032, which rounds to 40.03 pounds per day.
Answer: Yes, it is within the standard.
A pH of 7.4 falls between 6.5 and 8.5, so the finished water meets the secondary pH standard.
Answer: B) within the typical 0.2 to 2.0 mg/L range.
A residual of 0.9 milligrams per liter sits inside the 0.2 to 2.0 range utilities aim to keep in distribution, well below the 4.0 MRDL, so it is a normal distribution-system reading.
Answer: 120,000 mg/L.
A 1 percent solution is 10,000 mg/L, so a 12 percent solution is 12 × 10,000 = 120,000 milligrams per liter.
Answer: 83.4 lb/day.
Loading = 2.5 mg/L × 4.0 MGD × 8.34 = 10 × 8.34 = 83.4 pounds per day. Raising the flow while holding the dose raises the feed rate in proportion to the flow.
Answer: Chlorine residual drops as water travels through the pipes.
Chlorine keeps reacting with material in the pipes as water moves through the system, so the residual falls with distance and time. Measuring far out confirms that enough disinfectant is still present at the ends of the system, where it is lowest, not just at the plant.

Differentiation

Support

Provide the pounds formula and the chlorine-demand relationship on a printed reference card, and set up item 2 with the numbers already placed in the formula so students only carry out the multiplication. Allow calculators on every computation item.

Extension

Ask students to find the chlorine dose, in milligrams per liter, the plant would need at 3.0 MGD to feed exactly 100 pounds of chlorine per day, working backward through the pounds formula, and then use the chlorine demand from item 3 to estimate the residual that dose would leave and check that it stays below the 4.0 mg/L MRDL.

ELL supports

Provide a labeled diagram of the treatment train showing where chlorine is added, the contact tank, and a distribution-system sampling point. Pre-teach "dose," "residual," and "demand" with the diagram, and give a sentence frame for item 9: "The operator measures far out because ___."