Concentration

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# Concentration - PowerPoint PPT Presentation

Solutions. Concentration. Concentration Units. The amount of solute in a solution. Describing Concentration % by mass - medicated creams % by volume - rubbing alcohol ppm, ppb - water contaminants molarity - used by chemists molality - used by chemists. moles solute. =. Molarity.

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## PowerPoint Slideshow about 'Concentration' - megan

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Presentation Transcript

Solutions

Concentration

Concentration Units

• The amount of solute in a solution.
• Describing Concentration
• % by mass - medicated creams
• % by volume - rubbing alcohol
• ppm, ppb - water contaminants
• molarity - used by chemists
• molality - used by chemists

moles solute

=

Molarity

(

M

)

liters of solution

Molarity

PROBLEM: Dissolve 5.00 g of NiCl2•6 H2O (237.7 g/mol) in enough water to make 250 mL of solution. Calculate the Molarity.

Step 1: Calculate moles of NiCl2•6H2O

Step 2: Calculate Molarity

[NiCl2•6 H2O = 0.0841 M

What mass of oxalic acid, H2C2O4, is required to make 250. mL of a 0.0500M solution?

Step 1: Change mL to L:250 mL * 1L/1000mL = 0.250 L

Step 2: Calculate Moles

= (0.0500 mol/L) (0.250 L) = 0.0125 moles

Step 3: Convert moles to grams.

(0.0125 mol)(90.00 g/mol) = 1.13 g

Learning Check

How many grams of NaOH are required to prepare 400. mL of 3.0 M NaOH solution?

1) 12 g

2) 48 g

3) 300 g

mol solute

m of solution

=

kilograms solvent

Two Other Concentration Units

MOLALITY, m

% by mass

grams solute

grams solution

% by mass =

Percent Composition

• This is the mass of the solute divided by the mass of the solution (mass of solute plus mass of solvent), multiplied by 100.
• Example: Determine the percent composition by mass of a 100 g salt solution which contains 20 g salt.

(20 g NaCl / 100 g solution) x 100 = 20% NaCl solution

mass of solvent only

1 kg water = 1 L water

Molality

Calculating Concentrations

Dissolve 62.1 g (1.00 mol) of ethylene glycol in 250. g of H2O. Calculate m & % of ethylene glycol (by mass).

Calculate molality

Calculate weight %

Molality

• Find the molality of a solution containing 75 g of MgCl2 in 250 mL of water.

75 g MgCl2

1 mol MgCl2

95.21 g MgCl2

0.25 kg water

= 3.2m MgCl2

Molality

• How many grams of NaCl are req’d to make a 1.54m solution using 0.500 kg of water?

0.500 kg water

1.54 mol NaCl

1 kg water

58.44 g NaCl

1 mol NaCl

= 45.0 g NaCl

Dilutions

• Preparation of a desired solution by adding water to a concentrate.
• Moles of solute remain the same.

Dilution

• What volume of 15.8M HNO3 is required to make 250 mL of a 6.0M solution?

GIVEN:

M1 = 15.8M

V1 = ?

M2 = 6.0M

V2 = 250 mL

= 0.250 L

WORK:

M1 V1 = M2 V2

(15.8M)V1 = (6.0M)(0.250 L)

V1 = 0.095 L of 15.8M HNO3