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Quantitative Methods: Part 1. Solutions and Dilutions

Author(s): David R. Caprette, PhD

Example: Prepare 2 Liters 0.85% Sodium Chloride

With 1% defined as 1 gram per 100 ml, 0.85% is 0.85 grams per 100 ml. Since two liters is 20x the volume of 100 ml, we need 20 x 0.85 grams, or 17 grams NaCl. For this quantity, we can use a top loading balance or even a trip balance.

A typical electronic balance is accurate to one hundredth of a gram, which is sufficiently accurate for weighing out 17 grams. First we "tare" the instrument by placing a weigh boat onto the pan and setting it to "zero." We don't want to contaminate our chemical stocks, so we either clean the spatula or spoon before dipping it into the container, or we simply shake the chemical out onto the boat.

Suppose we tap out 16.97 grams of NaCl. Should we go to the trouble to get that last 0.03 gram? No, we are close enough. If it was necessary to be more accurate, we would describe the formula as something like 0.846% NaCl, or maybe 0.8495%. If there is some advantage to being that precise, we should exercise precision. Otherwise, trying to be too precise just wastes time.

Remember how to use significant digits? Seventeen grams means greater than 16.5 grams and less than 17.5 grams. If we wanted to be more accurate we would write "17.0" grams, meaning greater than or equal to 16.95 grams and less than or equal to 17.05 grams.