ABSTRAKPenelitian ini bertujuan untuk meningkatkan: (1) prestasi belajar siswa padamateri stoikiometri melalui penerapan metode pembelajaran Problem Solving, (2)kemampuan berpikir kritis siswa pada materi stoikiometri melalui metodepembelajaran Problem Solving.Penelitian ini merupakan Penelitian Tindakan Kelas (Classroom ActionResearch) yang dilaksanakan dalam dua siklus. Setiap siklus terdiri dari empattahapan yaitu: perencanaan, tindakan, observasi, dan refleksi. Subyek penelitianadalah siswa kelas X MIA 7 SMAN 1 Sukoharjo Tahun Pelajaran 2014/2015 yangtelah diberlakukan Kurikulum 2013. Pengumpulan data penelitian menggunakanteknik tes untuk kompetensi pengetahuan dan kemampuan berpikir krtis, dan non tesyaitu angket dan lembar observasi untuk kompetensi sikap dan lembar observasiuntuk kompetensi ketrampilan. Analisis data menggunakan teknik analisis deskriptifkualitatif.Hasil penelitian menunjukkah bahwa penerapan metode pembelajaranProblem Solving dapat meningkatkan prestasi belajar dan kemampuan berpikir krtitissiswa. Peningkatan prestasi belajar dapat dilihat dari hasil kompetensi pengetahuan,sikap dan ketrampilan. Ketuntasan belajar siswa pada kompetensi pengetahuanmemperoleh persentase dari 68,13% pada siklus I meningkat menjadi 81,57% padasiklus II. Ketercapaian aspek kompetensi sikap pada siklus I sebesar 77,95% danpada siklus II sebesar 86,19%. Ketercapaian aspek kompetensi ketrampilan padasiklus I sebesar 72,95% dan pada siklus II sebesar 84,14%. Untuk peningkatankemampuan berpikir kritis siswa pada prasiklus, siklus I, dan siklus II yaitu 41,01%,58,94% dan 76,28%.Kata kunci: Problem Solving, pretasi belajar, kemampuan berpikir kritisABSTRACTThe purpose of this research were to improve: 1) students achievement instoichiometry throught the application of learning methods of Problem Solving, 2)The critical thinking ability of students in stoichiometry through the application oflearning methods of Problem Solving.This research was a Classroom Action Research which was conducted intwo cycles. Every cycle of four stages that were: planning, action, observation, andreflection. This research subject is the students of class X MIA 7 SMAN 1 Sukoharjoyear 2014/2015 that have used Curriculum 2013. The collection of research data wasusing test techniques for the learning achievement, knowledge and ability to thinkcritically and not test is using questionnaire, the observation sheet for competence ofattitude, and the observation sheet for competence of skill. Data analysis was usingqualitative descriptive analysis technique.The result of this research was showing that the application of learningmethods of Problem Solving could improve learning achievement and criticalthinking ability of students. The improvement of learning achievement could be seenfrom the result of knowledge competence, attitude, and skill. On the competence ofknowledge, mastery learning students from 68,13% in the first cycle increased to81,57% in the second cycle. Aspect competence of attitude improvement from77,95% in cycle I to 86,19% in cycle II. Aspect competence of skill improvementfrom 72,95% in cycle I to 84,14% in cycle II. For the improvement of criticalthinking ability of students in a pre cycle, first cycle and second cycle that is 41,01%,58,94% and 76,28%.Key Word: Problem Solving, learning achievement, critical thinking ability