Lesson 4.1 · 7 min
Mass, volume, concentration, and error
Objective: Distinguish mass, volume, and concentration, convert units, and explain why every measurement carries error.
Mass is how much matter something contains, usually measured in grams or milligrams. Volume is how much space a liquid takes up, usually measured in liters or milliliters. These are two different things measured with two different tools, a scale for mass and a graduated container for volume. Concentration describes how much of something is dissolved into something else, usually as mass per volume, like milligrams per milliliter. Concentration is not the same as the total amount you have: a small vial with a high concentration can contain less total material than a large vial with a low concentration.
Research uses the metric system because it scales cleanly in units of ten. A gram is a thousand milligrams. A liter is a thousand milliliters. A milliliter is a thousand microliters. The most common beginner mistake is a decimal point error, moving a value by a factor of ten or a thousand without noticing. Writing units next to every number, every single time, is the simplest defense against this.
No measuring tool is perfectly exact. A scale has a limit to how finely it can distinguish weight. A pipette has small natural variation from one use to the next. This built-in imprecision is called measurement error, and it exists even when you do everything correctly. Match your tool to your task, calibrate equipment when the manufacturer recommends it, and be honest in your records about the precision your equipment can actually deliver, rather than writing down more decimal places than your tool can truly support.
A dilution is adding solvent to make a solution less concentrated; the amount of material stays the same while the volume grows. Write the calculation down before you touch anything, including units at every step, and label the new container immediately with its concentration, source lot, and date. A balance reads correctly only when level, out of a draft, and zeroed with the container already on the pan (taring). Draw pipette liquid slowly, hold it upright, and use a fresh tip whenever the source changes.
- Concentration:
- The amount of a substance present in a given volume of a mixture, commonly expressed as mass per volume.
- Measurement error:
- The natural, unavoidable difference between a measured value and the true value, caused by the limits of the tool or method used.
- Taring:
- Zeroing a balance with the empty container in place so the display shows only the mass of the contents.