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# 7. Four litres of an acid has pH 3. How many litres of pure water must be added to the 4 L to make a solution of pH 5?

**7. **Four litres of an acid has pH 3. How many litres of pure water must be added to the 4 L to make a solution of pH 5?

**8. **How much more intense is an earthquake of magnitude 6.3 than one of magnitude 3.9?