user: Question: What is Mean Free Path, when Vacuum Range is "Medium vacuum"?
Columns: ['Vacuum range', 'Pressure in hPa (mbar)', 'Molecules / cm 3', 'Molecules / m 3', 'Mean free path']
Types: ['text', 'text', 'text', 'text', 'text']
Sample row: ['Ambient pressure', '1013', '2.7 × 10 19', '2.7 × 10 25', '68 nm']
SQL:

assistant: SELECT "Mean free path" FROM "Table" WHERE "Vacuum range" = 'Medium vacuum';