Solutions and the pH Scale: Molarity, Dilution and Reading pH

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Science · Chemistry
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Page 2
Solutions and the pH Scale: Molarity, Dilution
and Reading pH
Science · Chemistry · Grades 10-11 · 50 minutes
Time needed
50 minutes (one class period)
Prep
Copy the 2-page worksheet (one per student).
Technology
None. A basic calculator is optional.
Student
independence
High. Students work alone, then check with a partner.
Copies to make
One 2-page worksheet per student
Materials
Pencils, Worksheet, Calculators (strongly recommended)
Goal for today: Students will calculate molarity and dilution volumes and use a pH table to classify substances and
compare how acidic they are.
Before the bell
☐ Copy the worksheet, one per student (2 pages).
☐ Write on the board: M = moles ÷ liters. Also write: pH below 7 is acidic, 7 is neutral, above 7 is basic.
☐ Keep the answer key on your desk. No chemicals, water or glassware are used; this is a paper-and-calculator
lesson.
Say this to start
Good morning! Today in chemistry you will work with solutions, which are mixtures that look the same all the way
through, such as salt water. First you will calculate how concentrated solutions are. Then you will read a table of
everyday substances to see which are acids and which are bases. There is no lab today; it is all paper work.

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Page 3
Run of show
Minutes
What you do
What students do
0–5
Take attendance. Hand out the worksheet. Ask students to
read the Key Ideas box and the pH table.
Read the box and the table silently.
5–13
Read this example aloud: 3.00 mol of salt in 1.50 L of
solution. Molarity = 3.00 ÷ 1.50 = 2.00 M. Then: a pH of 4
means the hydrogen ion concentration is 1 × 10⁻⁴ M.
Follow along and copy the two examples.
13–30
Tell students to work questions 1–5 (molarity and dilution).
Walk around and ask, “What are the moles? What are the
liters?”
Solve each problem and show work in the
boxes.
30–42
Tell students to work questions 6–10 using the pH table.
Remind them that each pH step is a factor of 10.
Read the table and answer the pH
questions.
42–47
Read the answers aloud from the key. Students check and
correct their work.
Check answers and fix mistakes.
47–50
Collect the worksheets and put them in a stack for the
teacher.
Hand in the worksheet.
If things go wrong
• If students get the wrong answer in the molarity problems, check that they changed milliliters to liters (divide
by 1000).
• If students ask about 10⁻⁴, tell them it is the same as 0.0001. The exponent size tells you the pH.
• If students say they have not learned pH, tell them the Key Ideas box and the table are enough to answer
every question.
• If the class finishes early, use the early finisher list.
Early finishers
• Pick one substance from the pH table that is not on the worksheet questions and write a sentence describing
whether it is acidic, basic or neutral.
• Explain in two sentences why a pH of 2 is much more acidic than a pH of 4.
Support and challenge
• Support: Complete questions 1, 2, 3, 6 and 7. Use the example as a model.
• Challenge: How many grams of NaCl (58.44 g/mol) are in 250 mL of a 0.400 M solution? Show your steps on
the back.

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Science · Chemistry
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Page 4
Notes for the teacher (you fill this in)
Jot a few lines before you leave. A full Daily Sub Report is in the free Sub Survival Kit at SubstitutePlans.com.
How far the class got / what to pick up next:
Students who were especially helpful:
Anything the teacher should know: