You need to measure the mass of an empty graduated cylinder first because it helps you determine the mass of only the liquid you're measuring.

Here's why:

* Calculating Liquid Mass: When you pour a liquid into the graduated cylinder, you're essentially adding to the total mass. To find the mass of the liquid alone, you need to subtract the mass of the empty cylinder from the combined mass of the cylinder and liquid.

Example:

1. Empty cylinder mass: 100 grams

2. Cylinder + liquid mass: 150 grams

Liquid mass = (Cylinder + liquid mass) - (Empty cylinder mass)

Liquid mass = 150 grams - 100 grams = 50 grams

Without knowing the mass of the empty cylinder, you wouldn't be able to accurately determine the mass of the liquid you've measured.