2cro4 2−(aq)+2h+(aq)⇌cr2o7 2−(aq)+h2o(l)write the equation for the equilibrium constant (k) of the reaction studied in this exercise.

Explanation:  

The equilibrium constant for a given reversible aqueous reaction is defined by the product ratio of the concentrations between the products and reactants:      

                                           aA + bB ⇄ cC + dD

where K: is the equilibrium constant, [C] and [D]: are the product concentrations, [A] and [B]: are the reactant concentrations and a,b,c,d: are the stoichiometric coefficients from the reaction.  

Therefore, based on the definition the equilibrium constant of our reaction is:    

2CrO₄²⁻(aq) + 2H₃O⁺(aq) ⇄ Cr₂O₇²⁻(aq) + 3H₂O(l)

Generally, the water concentration is omitted from the expressions.

I hope it helps you!

i think slash and

the formula mass of a molecule or a compound is the sum of the atomic weights of the atoms in its empirical formula

(that's what i know about formula mass)