## Biology Tutorial

*Concentrations *

#### Intro

The concentration of a solution is usually expressed as 1M, or 1 mole/L, where one mole is is the formula weight (grams) of a molecule dissolved in one liter of water at standard temperature and pressure. The capital M can be expressed as mM (millimoles), uM (micromoles), nM (nanomoles) and smaller.

Concentration is sometimes expressed as mass per unit volume (mg/L, ug/uL), or as a percentage. A v/v percentage is the number of parts of solute (ex vinegar) in 100 parts of the solvent (ex water). A w/v percentage is the weight of the solute (in milligrams, grams, kilograms, etc.) in 100 mL of solvent.

#### Sample Problem

Calculate the total number of grams of NaCl (salt) in a 200 mL w/v 10% aqueous NaCl solution.

w/v 10% = 10 g / 100 mL = x g / 200 mL

#### Solution

We know that the 200 mL solution is double or two times the amount of a 100 mL solution, therefore the solute will also be double. The total number of grams of NaCl solute is 20 g in a 200 mL w/v 10% aqueous NaCl solution.

10 g x 2 / 100 mL x 2 = 20 g / 200 mL

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