The Effect of The Problem-Based Learning (PBL) Model with a Multi-Representation Approach on Students’ Critical Thinking Skills in The Buffer Solution ConceptErni Mohamad, Mardjan Paputungan and Asnita DahibaE3S Web Conf., 400 (2023) 04001DOI: https://doi.org/10.1051/e3sconf/202340004001