Final answer:
The student's Chemistry question is about calculating equilibrium concentrations by assuming the change in concentration is negligible, simplifying the equilibrium expression and substituting back to solve for x and thereby find the actual concentrations of reactants and products.
Step-by-step explanation:
Understanding Equilibrium Concentrations
The student's question involves calculating equilibrium concentrations in a chemical reaction. When dealing with an equilibrium constant expression like ((2.0-x)(0.40-x))/((x)(3.0+x))=0.686, it is common practice to assume that if the equilibrium constant is not very large or very small, the change in concentration (represented by x) will not be significant enough to affect the initial concentrations. In other words, if K indicates only a slight extent of reaction, we can simplify calculations by setting x to zero, thereby making (0.78 - x) approximately equal to 0.78 and similarly for other terms.
Once we simplify the equilibrium expressions by assuming that x is negligible, we substitute these simplified terms back into the original equilibrium constant expression to find the value of x. By doing so, we can obtain the equilibrium concentrations without needing to solve a more complex quadratic equation. After finding the value of x, we can then calculate the exact concentrations of reactants and products at equilibrium.