Updated May 22, 2018 By Claire Gillespie
In a solution, the solute is the minor component that is dissolved in the solvent. For example, salt is the solute in a salt water solution, and isopropanol or ethanol is the solute in a rubbing alcohol solution. Before working out moles of solute, you need to understand what a mole is.
The number of moles of solute = mass of solute ÷ molar mass of solute, where mass is measured in grams and molar mass (defined as the mass of one mole of a substance in grams) is measured in g/mol.
A mole (abbreviated to mol) is a very large number used to measure units (atoms, electrons, ions or molecules), which is equal to 6.022 x 10^23 (the same number of particles as there are atoms in 12 grams of carbon-12). This is known as Avogadro’s number or the Avogadro constant.
The number of moles of solute = mass of solute ÷ molar mass of solute, where mass is measured in grams and molar mass (defined as the mass of one mole of a substance in grams) is measured in g/mol. If you don't know the mass of the solute, weigh it on a scale and record the value.
To find the molar mass of the solute, refer to a periodic table. If the solute is a single element, calculate the molar mass of that element. If it consists of a more than one element (i.e. a compound) calculate the molar mass of the compound.
Every element has a different molar mass. For example, one mole of sodium (Na) has a mass of 22.9898 g/mol. One mole of chlorine (Cl) is 35.4530 g/mol. If your solute is table salt (NaCl), it is a compound of sodium and chlorine. To find the molar mass of NaCl, you add the mass of each element. Work out 22.9898 + 35.4530 = 58.4538.
Say you create a solution with 200 grams of table salt. One mole equals the molar mass of the solute which is 58.4538 grams. Divide the mass of the solute by the molar mass to get the number of moles of solute. In this case, work out 200 ÷ 58 = 3.4483 moles of solute.
When you know moles of solute, you can work out molarity (M), the concentration of a solution expressed as the number of moles of solute per liter of solution. To work out molarity, you need to know the total volume of solution as well as the number of moles of solute. Divide the number of moles of solute by the number of liters of solution. For example, if you have 3.4483 moles of table salt in 10 liters of water, work out 3.4483 ÷ 100 = 0.0345. The molarity is 0.0345 M.
The percentage concentration of any solution is most commonly expressed as mass percent: Mass % of any component of the solution = Other methods are: Volume % of a component =
i.e. Mass by Volume percentage = Here's a point to be kept in mind : The concentration of a solution is most of the time expressed as the number of moles of solute present in 1 Liter of the solution (also called molarity ) (There are also other ways to express concentration. Please follow this link. ) EXAMPLE: (b) What is the molarity of a solution prepared by dissolving 15.0 g of sodium hydroxide in enough water to make a total of 225 mL of solution? Solution
Moles of NaOH = 15.0 g NaOH × #(1"mol NaOH")/(40.00"g NaOH")# = 0.375 mol NaOH
Volume = 225 mL × #(1"L")/(1000"mL")# = 0.225 L soln
Molarity = #(0.375"mol")/(0.225"L")# = 1.67 mol/L
Let's address the question for both percent concentration by mass and for percent concentration by volume. Percent concentration by mass is defined as the mass of solute divided by the total mass of the solution and multiplied by 100%. So, #c% = m_(solute)/(m_(solution)) * 100%#, where #m_(solution) = m_(solvent) + m_(solute)# There are two ways to change a solution's concentration by mass
Let's take an example to better illustrate this concept. Say we dissolve 10.0g of a substance in 100.0g of water. Our concentration by mass will be #c% = (10.0g)/(10.0g + 100.0g) * 100% = 9.09%# Now let's try doubling the mass of the solute; the new concentration will be #c% = (2 * 10.0g)/(2*10.0g + 100.0g) * 100% = 16.7%# However, if we keep the mass of the solute at 10.0g and doubled the mass of the solvent (in this case, water), the concentration will be #c% = (10.0g)/(10.0g + 2*100.0g) * 100% = 4.76%# The same is true for percent concentration by volume, which is defined as the volume of the solute divided by the total volume of the solution and multiplied by 100%. #c_(volume)% = V_(solute)/(V_(solute) + V_(solvent)) * 100%# It's easy to see that manipulating either the volume of the solute or the volume of the solvent (or both) would change the solution's percent concentration by volume.
There are two types of percent concentration: percent by mass and percent by volume. PERCENT BY MASS Percent by mass (m/m) is the mass of solute divided by the total mass of the solution, multiplied by 100 %. Percent by mass = #"mass of solute"/"total mass of solution"# × 100 % Example What is the percent by mass of a solution that contains 26.5 g of glucose in 500 g of solution? Solution Percent by mass = #"mass of glucose"/"total mass of solution" × 100 % = (26.5"g")/(500"g")# × 100 % = 5.30 % PERCENT BY VOLUME Percent by volume (v/v) is the volume of solute divided by the total volume of the solution, multiplied by 100 %. Percent by volume = #"volume of solute"/"total volume of solution"# × 100 % Example How would you prepare 250 mL of 70 % (v/v) of rubbing alcohol Solution 70 % = #"volume of rubbing alcohol"/"total volume of solution" × 100 %# × 100 % So Volume of rubbing alcohol = volume of solution × #"70 %"/"100 %"# = 250 mL × #70/100# = 175 mL You would add enough water to 175 mL of rubbing alcohol to make a total of 250 mL of solution.
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What concentration is calculated as the ratio of the moles of solute to the mass of the solvent?
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