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Accounting for Ions in a Model Water Softener 4 questions
Accounting for Ions in a Model Water Softener
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A class models an ion exchange cartridge. The cartridge contains fixed negative sites that initially hold Na+ ions. As Ca2+ ions pass through the cartridge, one Ca2+ can replace enough Na+ to keep the fixed sites electrically balanced. Assume these are the only ions the cartridge exchanges.
Question 1
How many Na⁺ ions must leave the cartridge for each Ca²⁺ ion it holds? Use the charges to explain.
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The class passes three equal portions through the same cartridge in sequence. It measures the amount of Ca2+ entering and leaving in each portion. The inlet amount is 0.50 mmol Ca2+ each time.
| Portion | Ca2+ leaving (mmol) |
|---|---|
| 1 | 0.10 |
| 2 | 0.25 |
| 3 | 0.50 |
Question 2
Calculate the amount of Na⁺ released from the cartridge into portion 2. Show how you used the calcium measurement and the exchange ratio.
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Question 3
Which interpretation best fits the three calcium readings?
- The cartridge removes more calcium from each successive portion.
- The cartridge removes the same amount of calcium each time.
- The cartridge removes less calcium from each successive portion.
- The cartridge adds calcium to portion 3.
Question 4
A student argues that removing calcium must lower the total amount of dissolved ions in the first portion. Consider only the Ca²⁺ and Na⁺ ions changed by the exchange. Use the ratio to evaluate the claim.
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