3 deleted 5 characters in body edited Apr 20 '15 at 13:00 Loong♦ 37.1k99 gold badges9292 silver badges196196 bronze badges Dimensional analysis is useless. TheThe correct answer is the one already given involving activities, which are dimensionless. Activities are defined as ratios. ForFor example a pressure activity (there are many kinds) is defined in terms of the ratio of the actual pressure of a gas divided by the reference pressure, often 1 atm atm or 1 bar bar.   In the present example, the activity is the ratio of the molality divided by the reference molality of 1 molal. ThisThis assumes ideal solutions, which is good enough if the solutions are dilute. IfIf the solution is not ideal one has to correct the molality for nonideality. Because of these complications, detailed discussions of activities are usually left for a course in physical chemistry. So while we use square brackets and molarities, we have to understand that we are really dealing with activities. By the way, the activity of a pure liquid or solid is 1, which is why $$[H_2O]$$$$[\ce{H2O}]$$, for example, is dropped from equilibrium calculations. Dimensional analysis is useless. The correct answer is the one already given involving activities, which are dimensionless. Activities are defined as ratios. For example a pressure activity (there are many kinds) is defined in terms of the ratio of the actual pressure of a gas divided by the reference pressure, often 1 atm or 1 bar.   In the present example, the activity is the ratio of the molality divided by the reference molality of 1 molal. This assumes ideal solutions, which is good enough if the solutions are dilute. If the solution is not ideal one has to correct the molality for nonideality. Because of these complications, detailed discussions of activities are usually left for a course in physical chemistry. So while we use square brackets and molarities, we have to understand that we are really dealing with activities. By the way, the activity of a pure liquid or solid is 1, which is why $$[H_2O]$$, for example, is dropped from equilibrium calculations. Dimensional analysis is useless. The correct answer is the one already given involving activities, which are dimensionless. Activities are defined as ratios. For example a pressure activity (there are many kinds) is defined in terms of the ratio of the actual pressure of a gas divided by the reference pressure, often 1 atm or 1 bar. In the present example, the activity is the ratio of the molality divided by the reference molality of 1 molal. This assumes ideal solutions, which is good enough if the solutions are dilute. If the solution is not ideal one has to correct the molality for nonideality. Because of these complications, detailed discussions of activities are usually left for a course in physical chemistry. So while we use square brackets and molarities, we have to understand that we are really dealing with activities. By the way, the activity of a pure liquid or solid is 1, which is why $$[\ce{H2O}]$$, for example, is dropped from equilibrium calculations. 2 deleted 19 characters in body edited Sep 22 '12 at 1:45 jonsca♦ 1,80477 gold badges2626 silver badges5252 bronze badges Dimensional analysis is useless. The correct answer is the one already given involving activities, which are dimensionless. Activities are defined as ratios. For example a pressure activity (there are many kinds) is defined in terms of the ratio of the actual pressure of a gas divided by the reference pressure, often 1 atm or 1 bar. In the present example, the activity is the ratio of the molality divided by the reference molality of 1 molal. This assumes ideal solutions, which is good enough if the solutions are dilute. If the solution is not ideal one has to correct the molality for nonideality. Because of these complications, detailed discussions of activities are usually left for a course in physical chemistry. So while we use square brackets and molarities, we have to understand that we are really dealing with activities. By the way, the activity of a pure liquid or solid is 1, which is why $$[H_2O]$$, for example, is dropped from equilibrium calculations. ---- Paul J. Gans  Dimensional analysis is useless. The correct answer is the one already given involving activities, which are dimensionless. Activities are defined as ratios. For example a pressure activity (there are many kinds) is defined in terms of the ratio of the actual pressure of a gas divided by the reference pressure, often 1 atm or 1 bar. In the present example the activity is the ratio of the molality divided by the reference molality of 1 molal. This assumes ideal solutions, which is good enough if the solutions are dilute. If the solution is not ideal one has to correct the molality for nonideality. Because of these complications detailed discussions of activities are usually left for a course in physical chemistry. So while we use square brackets and molarities, we have to understand that we are really dealing with activities. By the way, the activity of a pure liquid or solid is 1, which is why $$[H_2O]$$, for example, is dropped from equilibrium calculations. ---- Paul J. Gans  Dimensional analysis is useless. The correct answer is the one already given involving activities, which are dimensionless. Activities are defined as ratios. For example a pressure activity (there are many kinds) is defined in terms of the ratio of the actual pressure of a gas divided by the reference pressure, often 1 atm or 1 bar. In the present example, the activity is the ratio of the molality divided by the reference molality of 1 molal. This assumes ideal solutions, which is good enough if the solutions are dilute. If the solution is not ideal one has to correct the molality for nonideality. Because of these complications, detailed discussions of activities are usually left for a course in physical chemistry. So while we use square brackets and molarities, we have to understand that we are really dealing with activities. By the way, the activity of a pure liquid or solid is 1, which is why $$[H_2O]$$, for example, is dropped from equilibrium calculations. 1 answered Sep 12 '12 at 0:41 Paul J. Gans 3,2991111 silver badges1111 bronze badges Dimensional analysis is useless. The correct answer is the one already given involving activities, which are dimensionless. Activities are defined as ratios. For example a pressure activity (there are many kinds) is defined in terms of the ratio of the actual pressure of a gas divided by the reference pressure, often 1 atm or 1 bar. In the present example the activity is the ratio of the molality divided by the reference molality of 1 molal. This assumes ideal solutions, which is good enough if the solutions are dilute. If the solution is not ideal one has to correct the molality for nonideality. Because of these complications detailed discussions of activities are usually left for a course in physical chemistry. So while we use square brackets and molarities, we have to understand that we are really dealing with activities. By the way, the activity of a pure liquid or solid is 1, which is why $$[H_2O]$$, for example, is dropped from equilibrium calculations. ---- Paul J. Gans