Final answer:
John Dalton was the first chemist to perform truly quantitative experiments in chemistry by developing the atomic theory and calculating the relative masses of elements in compounds.
Step-by-step explanation:
Many argue that modern chemical science began when scientists started exploring the quantitative as well as the qualitative aspects of chemistry. For example, John Dalton was the first chemist to perform truly quantitative experiments. His atomic theory was an attempt to explain measurements that allowed him to calculate the relative masses of elements combined in various compounds, thus providing a quantitative description of the composition of substances.