5.3 Basic Molecular Calculations

Calculating the Mass of TRIS Required for a Solution

Let's run through the following example as a warm up:

Question: Calculate how much TRIS (Tris(hydroxymethyl)aminomethane) is required to make 250 ml of a 200 mM TRIS solution. The molecular mass of TRIS is 121.14 g/mol.

Step-by-Step Breakdown

Formula to use:

Mass (g) = Molarity (M) × Volume (L) × Molecular Mass (g/mol)

where:

• Molarity (M) is the concentration of the solution in moles per litre (mol/L).
• Volume (L) is the volume of the solution in litres.
• Molecular Mass (g/mol) is the molecular weight of the substance.

Step-by-Step Solution

1. Write down the given information:
   • Molarity (M) = 200 mM (millimolar) = 0.200 M (since 1 mM = 0.001 M)
   • Volume (V) = 250 ml = 0.250 L (since 1 L = 1000 ml)
   • Molecular Mass of TRIS = 121.14 g/mol
2. Plug the values into the formula:
   • Mass (g) = Molarity (M) × Volume (L) × Molecular Mass (g/mol)
   • Mass (g) = 0.200 × 0.250 × 121.14
3. Perform the calculation:
   • First, multiply the molarity and volume:
     • 0.200 × 0.250 = 0.050
   • Now, multiply by the molecular mass of TRIS:
     • 0.050 × 121.14 = 6.057
4. Final Answer:
   The mass of TRIS required is 6.06 g (rounded to two decimal places).