Start by calculating the number of moles in one gram of water, using periodic table data for hydrogen and oxygen: The moles of NaCl is provided, but you still need the number of moles of water, H 2O. Symbol: X or the lower-case Greek letter chi, χ, which is often written as a subscriptĬalculate Mole Fraction: X A = (moles of A) / (moles of A + moles of B + moles of C.)Įxample: Determine the mole fraction of NaCl in a solution in which 0.10 moles of the salt is dissolved in 100 grams of water.
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When this is done, the mole fraction is multiplied by 100%. Note some people express mole fraction as a percent (not common).
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Note that moles cancel out when calculating mole fraction, so it is a unitless value. The sum of all mole fractions adds up to 1. Mole fraction or molar fraction is the number of moles of one component of a solution divided by the total number of moles of all chemical species.
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How to Calculate Mole Fraction of a Solution You can set this up as an equation and solve for the unknown "x":ġ2 g iron / 100 g sample = x g iron / 250 g sample If you have 250 grams of nichrome, how much iron do you have?īecause the concentration is a percent, you know a 100-gram sample would contain 12 grams of iron. Mass percent is used for all sorts of solutions but is particularly useful when dealing with mixtures of solids or anytime physical properties of the solution are more important than chemical properties.Ĭalculate Mass Percent: mass solute divided by mass final solution multiplied by 100%Įxample: The alloy Nichrome consists of 75% nickel, 12% iron, 11% chromium, 2% manganese, by mass. Remember, the sum of all percentages of components in a solution must add up to 100% Simply use a scale to measure the mass of the solute and the final solution and express the ratio as a percentage. Mass percent composition (also called mass percent or percent composition) is the easiest way to express the concentration of a solution because no unit conversions are required. How to Calculate Mass Percent Concentration of a Solution If you have more precise measurements, don't forget to include the mass of solute in your calculation! Using 2 significant figures, it's the same thing. Now, you have 250 ml of water, which is about 250 g of water (assuming a density of 1 g/ml), but you also have 3 grams of solute, so the total mass of the solution is closer to 253 grams than 250. (1 mole / 74.6 g) * 3 grams = 3 / 74.6 = 0.040 molesĮxpress this as moles per kilogram solution. Then add them together to get the grams per mole for KCl.įor 3 grams of KCl, the number of moles is: Start by looking up the number of grams per mole of potassium and chlorine on a periodic table. Note that with aqueous solutions at room temperature, the density of water is approximately 1 kg/L, so M and m are nearly the same.Ĭalculate Molality: moles solute per kilogram solventĮxample: What is the molality of a solution of 3 grams of KCl (potassium chloride) in 250 ml of water?įirst, determine how many moles are present in 3 grams of KCl. Molality is used to express the concentration of a solution when you are performing experiments that involve temperature changes or are working with colligative properties. When you prepare a molar solution, avoid this problem by adding solvent to your solute to reach a specific volume. Note that I assumed dissolving the 6 grams of salt did not appreciably affect the volume of the solution. M = 0.62 moles NaCl / 0.50 liter solution = 1.2 M solution (1.2 molar solution) Now determine moles per liter of solution:
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It's one of the easiest units to calculate.Ĭalculate Molarity: moles solute per liter of solution ( not volume of solvent added since the solute takes up some space)Įxample: What is the molarity of a solution of 6 grams of NaCl (~1 teaspoon of table salt) dissolved in 500 milliliters of water?įirst, convert grams of NaCl to moles of NaCl. It is used when the temperature of an experiment won't change. Molarity is one of the most common units of concentration.