Final answer:
Adding enough 3 M H2SO4 to lower the pH to 4 involves calculating the required hydronium ion concentration using stoichiometry, considering sulfuric acid's diprotic nature and the initial conditions of the solution.
Step-by-step explanation:
If you added only enough 3 M H2SO4 to bring the pH down to 4, you would be adding a strong acid to the solution. To calculate the amount of acid needed, you would need to know the starting pH of the solution and its buffering capacity. Assuming the solution is initially at a higher pH and not strongly buffered, the addition of a small volume of concentrated sulfuric acid would increase the hydronium ion concentration, thus lowering the pH.
The exact amount of acid to be added can be determined using stoichiometry and the formula for pH which is pH = -log[H3O+]. Considering sulfuric acid is diprotic (releases two H+ ions per molecule), this will impact the calculation as well.
For example, if you started with a neutral solution (pH 7), progressing to a lower pH would follow the trend as in the provided information; as the acidity increases, the pH decreases accordingly. Applying the correct stoichiometry and accounting for the acid's molarity, you can calculate the volume of 3 M H2SO4 needed to achieve the desired pH. Remember that the presence of buffers or other reactions in the solution can affect this process.