Over the years there exists gender inequality in science achievement among senior secondary school students the world over. It is observed that the males score higher than the females in science and science- related examinations. This has created a big psychological alienation or depression in the minds of female students towards science and science- related subjects. This paper attempted to find out if sex differences exist in calculating reacting masses from a set of chemical equations among secondary school students in Makurdi metropolis. A Calculation Achievement Test (CAT) was administered to thirty students randomly selected from Government secondary school, North-Bank, Makurdi. The t-test statistic for independent samples was used to analyse the data obtained. The study established that boys performed better than girls on the achievement test. Recommendations to address the gender disparity in students' performance in chemistry were made. These include: teaming up of chemistry and mathematics teachers to ensure integrative learning, transfer and application of knowledge among the females by giving them more attention/time during classes. (Contains 2 tables.)