On your first day working in a laboratory, you are told to make up 1 Liter of a 0.5 Molar Tris solution with a pH of 7.4. You measure out the appropriate amount of Tris base and dissolve it in water. You then measure the pH and find out that it has an initial pH of 11. You add some concentrated HCl, and the pH begins to drop. However, as you approach the target pH, you find that you need to add large amounts of HCl to lower the pH even a small amount. If the molecular weight of Tris base is 121.14, how many grams did you use

60.57g of TRIS did you use

Explanation:

The concentration of the Tris solution buffer is 0.5 molar (That is, 0.5 moles per liter). As you need to make 1L, you need to have in the solution 0.5 moles of TRIS.

To convert these moles of TRIS to grams you need to use the molecular weight of TRIS base (121.14g/mol), thus:

0.5 moles TRIS * (121.14g / mol) =

n+h→nh3

we must remember that nitrogen and hydrogen are both diatomic molecules in their standard gas form. this adjusts the equation to

n2+h2→nh3

now we need to adjust coefficients in order to balance the atoms on each side of the equation. currently we have 2 atoms of nitrogen and 2 atoms of hydrogen on the reactant side and 1 atom of nitrogen and 3 atoms of hydrogen on the product side.

we can balance the hydrogens by placing a coefficient of 2 in front of ammonia and a coefficient of 3 in front of the hydrogen.

n2+3h2→2nh3

this gives us 6 hydrogen on each side and coincidentally the nitrogens now equal 2 on each side.