 Mass fraction is defined as the ratio between the mass of the constituent of interest and the total mass of the solution and we use the w slash w symbol to indicate that is a mass fraction. From the solutions video we talk about sterling siever being a solution with siever being the most abundant about 92.5% of the alloy siever and other metals like nickel and copper which take the remaining 7.5%. Now you will often hear people saying that sterling siever is an alloy of siever containing 92.5% by weight of siever and 7.5% by weight of other metals. This term by weight following a percentage is one way to represent a concentration of something in a solution and it is mass fraction or mass percent. It's also worth noticing that these alternative concentration definitions are not strictly a representation of concentration because it's not divided by an amount of volume but it's per an amount of masses instead. So a sterling siever alloy contains 92.5% by weight of siever and 7.5% by weight of other metals means that for every 100 grams of the alloy there you get 92.5 grams of siever and 7.5 grams of other metals. You can write that in term of mass fraction is sterling siever contains 92.5% w slash w so that means by weight and other metals 7.5% w slash w or by weight. Density. Remember how the mass concentration is the mass of the solute per an amount of volume of the solution? Well density is similar but it does not regard the specific constituents. It only concerns the total mass and the total volume of the solution. So say you have a solution A, its volume is say 2 liters and the mass of this whole solution all together is clothing the container of course is 2.2 kilograms. Then the density of this whole solution is simply mass developed by volume equal to 1.1 kilograms per liter. So it means that every liter of this solution weighs 1.1 kilograms. It does not give you any specific information on the mass of the solvent or a solute. It only gives you the mass of the whole solution together. Molality. Okay another useful quantity to know is molality. Molality is defined as the number of moles of the constituent of interest divided by the mass of the solvent is specifically in kilograms. Hence you also see people say that molality is equal to the number of moles over kilograms of solvent. So the simple is little b. For example this solution is made of 2.7 moles of potassium nitrate KNO3 and 5 kilograms of water. So the molality of this potassium nitrate solution is 2.7 moles divided by 5 kilograms equal to 0.54 moles per kilogram or 0.54 molar. Simple is little m in contrast to molar with his simple being big m. So molality is used when molality is not constant and this can be due to many scenarios when the temperature changes for example which would result in changes in volume but but because the amount of constituent stayed the same consequently molality will not change. Meanwhile the mass of the solvent is constant at all temperatures. So molality is maintained when temperature changes. This will come in handy when we talk about colligative properties which will usually involve temperature or pressure changes.