Which element is not found in the skeleton of amino acids?

Serine. Serine is non-essential amino acid supplied from food or synthesized by the body from a number of metabolites, including glycine. Serine is found in soybeans, nuts (especially peanuts, almonds, and walnuts), eggs, chickpeas, lentils, meat, and fish (especially shellfish).

Explanation:

Molality is defined as the number of moles of solute in 1 kg of solvent.
In this case the solvent is water, therefore we need to find the number of moles of KBr in 1 kg of water.
Number of moles of KBr - 2.10 g / 119 g/mol  = 0.0176 mol
mass of water - 897 g
number of moles of KBr in 897 g - 0.0176 mol
Therefore number of KBr moles in 1000 g - 0.0176 mol / 897 g x 1000 g = 0.0196 mol/kg
therefore molality of KBr is 0.0196 mol/kg