To measure the concentration of an aqueous solution of , an analytical chemist adds strong acid to a sample of the solution and titrates it to the endpoint with of potassium permanganate solution. The balanced chemical equation for the reaction is:

The question is incomplete, the complete question is shown in the image attached.

7.69%

Explanation:

The balanced equation of this reaction is shown in the image attached to this answer.

The reaction is redox because we can notice a change in the oxidation number of species from left to right in the reaction equation.

For instance, the oxidation number of oxygen was changed from -1 in H2O2 to zero in O2. Hence H2O2 was oxidized.

To calculate the percentage of H2O2 in the sample;

number of moles of KMnO4 =  137 ml/1000 * 0.230 M = 0.03151 moles of KMnO4

From the reaction equation;

2 moles of permanganate reacts with 5 moles of H2O2

0.03151 moles of permanganate reacts with  0.03151 moles * 5/2 = 0.079 moles of H202

Mass of H2O2 reacted = 0.079 moles of H202 * 34 gmol-1 = 2.69 g

Hence % by mass of H2O2 in the sample = 2.69 g/35 g * 100 = 7.69%

since the atomic number is the number of protons in the nucleus, as we move to the left on the periodic table, the electrostatic pull between the nucleus and the valence electron shell decreases. as we move down a group (vertical column), the valence shell energy level gets further and further from the nucleus.