A protein was previously determined to contain 16.2 wt% nitrogen. A 691 μL aliquot of a solution containing the protein was digested in boiling sulfuric acid. The solution was made basic and the liberated NH3 was collected in 10.00 mL of 0.0390 M HCl . A volume of 4.16 mL of 0.0155 M NaOH was required to react with the excess, unreacted HCl . Calculate the protein concentration of the solution in units of milligrams per milliliter.

6.60mg N / mL

Explanation:

All nitrogen is converted in NH₃ that react with the HCl, thus:

NH₃ + HCl → NH₄⁺ + Cl⁻

In the problem, the excess of HCl reacts with NaOH, thus:

HCl + NaOH → NaCl + H₂O

The moles of NaOH = Moles of HCl in excess is:

4.16mL = 4.16x10⁻³L * (0.0155mol / L) = 6.448x10⁻⁵mol HCl in excess

Initial moles of HCl are:

10x10⁻³L * (0.0390mol / L) = 3.9x10⁻⁴moles HCl

That means the moles of HCl that reacts = Moles of NH3 = Moles of N are:

3.9x10⁻⁴ moles - 6.448x10⁻⁵moles = 3.2552x10⁻⁴ moles N.

To convert these moles to grams we need to use molar mass of N = 14.01g/mol:

3.2552x10⁻⁴ moles N * (14.01g/mol) = 4.56x10⁻³g * (1000mg / g) =

4.56mg of N

And volume in mL is:

691 μL * (1mL /  1000μL) = 0.691mL

Concentration in milligrams per mililiter is:

4.56mg N / 0.691mL =

chemical changes occur when a substance combines with another to form a new substance, called chemical synthesis or, alternatively, chemical decomposition into two or more different substances. these processes are called chemical reactions and, in general, are not reversible except by further chemical reactions. some reactions produce heat and are called exothermic reactions and others may require heat to enable the reaction to occur, which are called endothermic reactions. understanding chemical changes is a major part of the science of chemistry.