Problem: The mass per unit volume of a pure substance is defined to be its density. Calculate the volume of $1.2 \mathrm{~kg}$ of a liquid with a measured density of $639 . \frac{\mathrm{g}}{\mathrm{L}}$. Be sure your answer has a unit symbol and the correct number of significant digits

Step 1 ：Given that the mass of the liquid is 1.2 kg and the density is 639 g/L.

Step 2 ：First, we need to convert the mass from kilograms to grams. We know that 1 kilogram is equal to 1000 grams. So, we multiply the given mass by 1000 to convert it to grams: \(1.2 \, \text{kg} = 1.2 \times 1000 = 1200 \, \text{g}\).

Step 3 ：Next, we use the formula for volume which is mass divided by density: \(\text{Volume} = \frac{\text{Mass}}{\text{Density}}\).

Step 4 ：Substitute the given values into the formula: \(\text{Volume} = \frac{1200 \, \text{g}}{639 \, \text{g/L}} = 1.8779342723004695 \, \text{L}\).

Step 5 ：However, the problem asks for the answer with the correct number of significant digits. The given values have 2 and 3 significant digits. Therefore, our answer should also have 2 significant digits. So, we round off the volume to 2 significant digits: \(1.8779342723004695 \, \text{L} \approx 1.88 \, \text{L}\).

Step 6 ：Final Answer: The volume of the liquid is \(\boxed{1.88 \, \text{L}}\).