Standard solutions

• Chemists routinely prepare solutions needed for analysis, whose concentrations are known precisely
• These solutions are termed?standard solutions
• They are made as accurately and precisely as possible using three decimal place balances and volumetric flasks to reduce the impact of measurement uncertainties
• The steps are:

Volumes & concentrations of solutions

• The?concentration?of a solution is the amount of?solute?dissolved in a?solvent?to make 1 dm^3?of??solution
  • The solute is the substance that dissolves in a solvent to form a solution
  • The solvent is often water
• A?concentrated?solution is a solution that has a?high?concentration of solute
• A?dilute?solution is a solution with a?low?concentration of solute
• Concentration is usually expressed in one of three ways:
  • moles per unit volume
  • mass per unit volume
  • parts per million

Moles per unit volume

• The formula for expressing concentration using moles is:

• You must make sure you change cm^3?to dm^3??(by dividing by 1000)
• The relationships can be expressed using this formula triangle:

To use the concentration formula triangle cover the one you want to find out about with your finger and follow the instructions

Answer:

Step 1: Use the formula triangle to find the number of moles of NaOH needed

number of moles? =?concentration (mol dm^-3)?x?volume (dm³)??

n = 0.200?mol dm^-3?x 0.250?dm³

n =?0.0500 mol

Step 2:?Find the molar mass of NaOH

M?= 22.99 + 16.00 + 1.01 = 40.00 g mol^-1

Step 3: Calculate the mass required

mass = moles x molar mass

mass =? 0.0500 mol x 40.00 g mol^-1?? =?2.00 g

Mass per unit volume

• Sometimes it is more convenient to express concentration in terms of mass per unit volume
• The formula is:

• To change a concentration from mol dm^-3?to g dm^-3
  • Multiply the moles of solute by its molar mass

mass of solute (g) = number of moles (mol) x molar mass (g mol^-1)

Parts per million

• When expressing extremely low concentrations a unit that can be used is?parts per million?or?ppm
• This is useful when giving the concentration of a pollutant in water or the air when the absolute amount is tiny compared the the volume of water or air
• 1 ppm?is defined as
  • A mass of?1 mg?dissolved in?1 dm³?of water
• Since 1 dm³?weighs 1 kg we can also say it is
  • A mass of?1 mg?dissolved in?1 kg?of water, or 10^-3?g in 10³?g which is the same as saying the concentration is?1 in?10^6?or?1 in a million

Answer:

Step 1: calculate the total mass in mg assuming 3ppm(1 litre is the same as 1 dm³)

3 x 2.5 x 10^6?= 7.5 x 10^6?mg

Step 2: convert the mass into kilograms (1 mg = 10^-6?kg)

7.5 x 10^6??x 10^-6??kg =?7.5 kg