Lemon juice is pH 2; black coffee is pH 5. How much more acidic (higher $H^{+}$ concentration) is the lemon juice?

B. 1000×, ten per unit

$10^{3} = 1000\times$. Logarithmic scales compress huge ranges into small numbers, the same trick as decibels and earthquake magnitudes. Misreading pH differences as linear is off by orders of magnitude.