Final answer:
At pH 6.0, the major species present are sodium citrate and sodium hydrogencitrate due to the dissociation constants of citric acid and the pH adjustment using 1 M NaOH. These species contribute to the buffer system that stabilizes the pH of the solution.
Step-by-step explanation:
When equal volumes of 1 M solutions of citric acid, sodium citrate, sodium hydrogencitrate, and sodium dihydrogencitrate are combined and the pH is adjusted to 6.0 using 1 M NaOH, the major species present in the solution depend on the dissociation constants (pKa) of citric acid (a triprotic acid) and the dominant forms at pH 6.0. Citric acid has pKa values that span acidic to neutral pH levels. At a pH of 6.0, the first two acidic protons have dissociated, and the dominant species are likely to be sodium citrate (Na3C6H5O7) and sodium hydrogencitrate (Na2H2C6H5O7), due to the deprotonated states being more favorable at this pH level.
Since the pH is being adjusted using 1 M NaOH, a strong base, it will react with any remaining acidic protons to form water and further deprotonate citric acid and its derivatives.
However, due to buffering capacity of the citric acid system, the pH tends to stabilize around 6.0, where hydrogen citrate and citrate forms are present. The buffer system created by the presence of these species allows the solution to resist drastic changes in pH when small amounts of base, like NaOH, are added.