Calculate the actual molarity of a solution due to dissolving 20.0 grams of NaOH in sufficient water to yield a 482 cm3 solution.
Solution
Molarity is surely an expression of the moles of solute (NaOH) per liter of solution (water). To be effective this problem, you will be able to determine the quantity of moles of sodium hydroxide (NaOH) and then convert cubic centimeters into liters. You can refer to the Worked Unit Conversions if you need more help.
Step 1 Calculate the number of moles of NaOH that are in 20.0 grams.
Look up the atomic masses for the elements in NaOH from the Periodic Table. The atomic masses are found to be:
Na is 23.0
H is 1.0
O is 16.0
Plugging these values:
1 mol NaOH weighs 23.0 g + 16.0 g + 1.0 g = 40.0 g
So the number of moles in 20.0 g is:
moles NaOH = 20.0 g × 1 mol/40.0 g = 0.500 mol
Step 2 Determine the volume of solution in liters.
1 liter is 1000 cm3, so the volume of solution is: liters solution = 482 cm3 × 1 liter/1000 cm3= 0.482 liter
Step 3 Determine the molarity of the solution.
Simply divide the number of moles by the volume of solution to get the molarity:
molarity = 0.500 mol / 0.482 liter
molarity = 1.04 mol/liter = 1.04 M
Answer
The molarity of a solution made by dissolving 20.0 g of NaOH to make a 482 cm3 solution is 1.04 M
Dilution is known as a reduction inside the concentration of a chemical substance (gas, vapor, solution). It’s the process of decreasing the concentration of a solute in solution, usually simply by mixing with more solvent. To actually dilute a solution means to increase the solvent without the addition of more solute. The resulting solution is completely mixed in order to make sure that all parts of the solution are identical.
The same direct relationship applies to gases and vapors diluted in air for example. Even though, thorough mixing of gases and vapors may not be as easily accomplished.
To illustrate, if there are 10 grams of salt (the solute) dissolved in 1 litre of water (the solvent), this particular solution has a certain salt concentration/molarity. If one adds 1 litre of water to this solution the salt concentration is reduced. The diluted solution still contains 10 grams of salt/(0.171 moles of NaCl).
Mathematically this relationship is usually shown in the equation:
Where:
C1 = Concentration/molarity 1
V1 = Volume 1
C2 = Concentration/molarity 2
V2 = Volume 2
Molarity is the the amount of moles of solute dissolved in one litre of solution. The units of molarity are moles per litre ie moles of solute per litre of solution.
The equation for molarity is: (moles of solute)/(litres of solution)
Eg 3 moles per litre or 3 mol L-1
Additionally, volume can be measured in cubic decimeters: dm3 which are equal to litres.
Eg 3 moles dm-3 is the same as 3 mol L-1
A concentration gives the quantity of any dissolved compound (solute) inside a solution. Typically the molar concentration or molarity (M) reports the quantity of moles of the solute in one liter (L) of the solution. As an example, the molarity of 160 g of sodium perchlorate (NaClO4) dissolved in 400 ml of distilled water will be calculated.
Instructions
Things You’ll Need
- Periodic table of the elements
- Calculator:
Step 1:
Obtain the atomic weights of the elements that compose the dissolved compound on the periodic table of the elements. From the example, the atomic weights of sodium (Na), chlorine (Cl) and oxygen (O) are 23, 35.5 and 16 respectively.
Step 2:
Include the atomic weights of all atoms in the molecule to compute its molecular mass.
In this example, the molecular mass of NaClO4 is 23 35.5 (4 x 16) = 122.5 g/mole.
Step 3:
Divide the mass of dissolved compound by its molecular mass to determine the number of moles. In the example, the quantity of moles of NaCO4 is 160 g / 122.5 g/mole = 1.3 moles.
Step 4:
Divide the volume of the solution (in ml) by 1,Thousand to change it to liters (L). In the example, 400 ml will become 0.4 L.
Step 5:
Divide the quantity of moles of the dissolved compound by the solution volume to calculate the molar concentration. In the example, the molar concentration is 1.3 moles / 0.4 L = 3.3 M.
