Teaching methods can help learners to develop problem-solving skills and enhance their achievement in stoichiometry. Process oriented guided inquiry learning (POGIL) is one of the teaching methods that promote problem-solving skills because it provides opportunities for learners to work with many formulae through many steps in solving problems such as stoichiometric calculations of number of moles and concentration of solutions. To understand how POGIL can improve learners’ achievement and develop problem-solving skills, four grade 11 physical sciences classes of mixed gender and multicultural black learners were purposefully and conventional sampled from four different township schools in Pretoria, South Africa. Through pre- and post-test case study and lesson observations, two different independent groups (POGIL group and lecture group) were included in the study. POGIL group constituted 48 students, while lecture group 62 students taught by their respective teachers at their schools for three weeks using English second language. The results from the pre-test suggest that learners in all the four classes lacked problem-solving competencies in solving both the low-order and the high-order stoichiometry questions. According to the research interpretation, lesson observations of POGIL were active learning while lecture method was passive learning. The post-test results indicate statistically significant greater problem-solving competencies in POGIL group than in the lecture group. The study recommends the use of POGIL in teaching stoichiometry.