Preparing a Solution from a Solid
ex. What mass of sodium hydroxide is
required to make 500.0 mL of 0.200 mol/L solution?
m = ? n = cV n
= m/M
V = 0.5000L = 0.200 mol/L(0.5000L) m = 0.100 mol(40.00 g/mol)
c = 0.200 mol/L = 0.100 mol = 4.00 g
M = 40.00 g/mol
Procedure:
- Complete calculations to determine mass of solid solute required.
- Mass out the solid solute on the electronic scale.*
- Add about 3 cm of distilled H2O to an appropriately sized volumetric flask.**
- Place the solid solute into the volumetric flask, using a powder funnel.
- Dissolve the solid solute by swirling the flask.
- Fill the flask to the line*** with distilled water and label.****
- Top up to line when solution reaches room temperature.*****
Safety:
goggles; gloves and apron when appropriate
Tips and Tricks:
* If using a
hygroscopic solid (a substance that picks up moisture from the air),
like sodium hydroxide, measure and place the lid back on the stock
bottle asap.
** A volumetric flask is used because it has a high degree of accuracy. Other measuring equipment that measures volume accurately includes graduated cylinders, pipettes and burettes. Equipment that measures volume poorly includes beakers and Erlenmeyer flasks.
***
A volumetric flask has a line etched into the neck too show you where
to fill to attain the desired volume. You must take into consideration meniscus and parallax when filling.
**** Label should include the your name, as well as the chemical's name, concentration and the date it was prepared.
*****
Many solutions get hot during the dissolution process. When the
solution cools down, the solution level in the flask will drop so you
must top it up with distilled water.
Introducing a New Equation
Whether you place 1 mol of a salt into a teacup of water or into a bathtub of water you still have 1 mol of the compound. Therefore,
Introducing a New Equation
Whether you place 1 mol of a salt into a teacup of water or into a bathtub of water you still have 1 mol of the compound. Therefore,
nconcentrated
= ndilute
and since n = cV
cconcentratedVconcentrated = cdiluteVdilute
or simply written by replacing the letters with numbers,
c1V1 = c2V2
This equation will be useful in our next calculation.
Preparing a Solution from a Stock Solution
ex.
What volume of stock hydrochloric acid (12.0 mol/L) is required to make 1.00 L
of 0.150 mol/L solution?
V1
= ?
c1V1 = c2V2
c1
= 12.0 mol/L 12.0 mol/L(V1)
= 0.150 mol/L(1.00 L)
V2
= 1.00 L V1 = 0.0125
L
c2
= 0.150 mol/L
= 12.5 mL
We always convert to mL at the end because all our equipment measures in mL.
Procedure:
- Complete the calculation to determine the volume of stock solution required.
- Use a graduated cylinder to measure out the stock solution.
- Add about 3 cm of dH2O to the flask.
- Place the stock solution into a volumetric flask (1.00 L), using a funnel.
- Dissolve the stock solution.
- Fill the flask to the line and label.
- Top up to line when solution reaches room temperature.
Safety: goggles, gloves, apron, fume hood
Homework:
Practice, p. 136 # 20, 21, 22
Section Questions, p. 137 # 1-6
Answers: