Chemistry · Some Basic Concepts Of Chemistry · NEET
It is a method to convert a measurement from one unit to another without changing its actual value. You multiply the given quantity by a fraction that equals 1, such as (2.54 cm / 1 inch). Because the fraction equals 1, the real amount does not change, but the unit does. In NCERT this same method is called the factor label method, unit factor method, or dimensional analysis. All three names mean the same thing.
Look at the unit you want to remove and the unit you want to keep. Write the conversion fact as a fraction so that the unit you want to cancel is placed opposite (top vs bottom) to where it already is, and the unit you want in your answer stays. Example: to change 3 inch to cm, the given 'inch' is on top, so put 'inch' on the bottom of your factor: 3 in x (2.54 cm / 1 in) = 7.62 cm. The 'in' cancels and 'cm' remains.
Because both parts of the fraction are the same real length, mass, or time. For example 2.54 cm and 1 inch are the exact same length, so 2.54 cm / 1 in = 1. Multiplying any number by 1 does not change it. This is why the method is safe: only the unit changes, never the physical quantity.
Start from 1 m = 100 cm, so (1 m / 100 cm) = 1. For volume you must cube the whole factor: (1 m / 100 cm)^3 = 1 m^3 / 10^6 cm^3. So to change cm^3 to m^3 you multiply by (1 m^3 / 10^6 cm^3). NCERT shows this exact idea when it converts 2 L of milk to m^3. Remember: for area square the factor, for volume cube it.
Yes. NCERT converts 2 days to seconds in one line: 2 day x (24 h / 1 day) x (60 min / 1 h) x (60 s / 1 min) = 172800 s. Each factor cancels one unit and hands over the next, until only seconds are left. Line up the factors so each unwanted unit cancels the one before it.
You use unit reasoning with density. Take 1 L of solution, find its mass using density (mass = density x volume), subtract the mass of solute to get mass of solvent, then divide moles of solute by kg of solvent. Every step is really a unit conversion (mL to g using g/mL, g to kg by 1000). This is a very common NEET calculation, so practice the density step carefully.
In which of the following is the number of water molecules maximum?
The density of a 1 M solution of a compound X is 1.25 g mL^-1. The molality of the solution is (molar mass of X = 85 g mol^-1)
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Yes. NCERT lists all three names for the same method of converting units by multiplying with fractions that equal 1. In the exam any of these names may be used, so treat them as identical.
Square the whole factor for area (units of length^2) and cube it for volume (units of length^3). For example (1 m / 100 cm)^3 converts cm^3 to m^3. Forgetting to cube is a common mistake.
Almost every numerical in Some Basic Concepts of Chemistry, like mole, molarity, molality, and density problems, needs you to convert units correctly. Cancelling units also lets you check if your setup is right before you calculate, which saves marks and time.
Before putting in numbers, cancel the units and see if only your target unit remains. If the leftover unit is wrong, your unit factors are set up incorrectly. This quick check catches many silly errors in the exam.
