ANS:B - molar concentration

Isotonic solutions must have the same molar concentration. Isotonic solutions are solutions that have the same concentration of solute particles. This ensures that there is no net movement of water across a semi-permeable membrane when the solutions are separated by the membrane. In biological contexts, isotonic solutions are often used to maintain the osmotic balance of cells and tissues. Therefore, it's the molar concentration of solute particles that must be the same in isotonic solutions, not viscosity, normality, or critical temperature. Molar concentration, also known as molarity (denoted as "M"), is a measure of the concentration of a solute in a solution. It represents the number of moles of solute dissolved in one liter of solution. Molar concentration is a fundamental concept in chemistry and is widely used in various applications, including chemical reactions, stoichiometry, and solution preparation. The formula to calculate molarity is: 𝑀𝑜𝑙𝑎𝑟𝑖𝑡𝑦(𝑀)=Number of moles of soluteVolume of solution (in liters)Molarity(M)=Volume of solution (in liters)Number of moles of solute​ In this formula:

• The "number of moles of solute" refers to the amount of solute substance present in the solution, measured in moles.
• The "volume of solution" refers to the total volume of the solution in which the solute is dissolved, measured in liters.

For example, if you have 0.5 moles of sodium chloride (NaCl) dissolved in 1 liter of water, the molarity of the solution would be calculated as: 𝑀=0.5 mol1 L=0.5 MM=1L0.5mol​=0.5M This means that the molarity of the solution is 0.5 moles of NaCl per liter of solution. In other words, for every liter of solution, there are 0.5 moles of NaCl dissolved in it. Molar concentration is important because it allows chemists to accurately describe and quantify the concentration of solutions, which is crucial for various chemical reactions, analytical techniques, and industrial processes. It provides a standardized way to express the amount of solute in a given volume of solution, making it easier to compare and manipulate different solutions in laboratory and industrial settings.