The objectives of the research were to identify the difficulty level of various types of chemical representation for students and to examine the effects of two different models of teaching and learning chemistry (problem-solving based on multiple representations and problem-solving) on the students' cognitive achievement in chemical equilibrium. This research was a quasi-experimental research with posttest only design. The research was conducted in two different 11th-grade groups in a senior high school. The research was carried out with two groups: an experiment group (problem-solving based on multiple representations model) (26 students) and a control group (problem-solving model) (24 students). The research finding shows that there is a significant difference in the students' mean cognitive learning scores in chemical equilibrium among the two groups. The results showed that the score of the students' cognitive achievement in the experiment group was better than control group.