CALCULATION OF MOLAR SOLUTION.pptx

Calculating molar solutions a document prepared by Bright Kumwenda

1. CALCULATION OF MOLAR SOLUTION Mass Volume Concentratio n

2. Describe how to calculate mass using mole concept Describe how to dilute a molar solution from a stock solution Describe the calculation of molar volume of a gas at Standard Temperature and Pressure(STP) and at Room Temperature and Pressure(RTP) Objectives

3. The mole (or mol) represents a certain number of objects. The amount of a substance that contains the same number of entities as there are atoms in 12 g of carbon-12. Exactly 12 g of carbon-12 contains 6.02x 10 23 atoms.

4. One mole of H 2 O molecules contains 6.022 x 10 23 molecules. 1 mole contains 6.02 x 10 23 entities (Avogadro’s number) Mass of a mole of particles = mass of 1 particle x 6.022 x 10 23

5. The mass of an atom in amu is numerically the same as the mass of one mole of atoms of the element in grams. One atom of sulfur has a mass of 32.07 amu . One mole of S atoms has a mass of 32.07 g.

6. • For compounds, the molecular mass (in amu ) is numerically the same as the mass of one mole of the compound in grams. Calculate the molecular mass of a compound as the sum of the atomic masses of its elements. • Molecular mass H 2 O = (2 x atomic mass of H) + atomic mass of O = 2(1.008 amu ) + 16.00 amu = 18.02 amu • So, one mole of water (6.022 x 10 23 molecules) has a mass of 18.02 g.

7. •The molar mass ( M ) of a substance is the mass of one mole of entities (atoms, molecules, or formula units) of the substance. • Molar mass has units of grams per mole (g/mol)

8. •Use the molar mass of an element or compound to convert a given number of moles to mass: •We can do the reverse and convert any mass in grams to the number of moles: •Use Avogadro’s number to convert moles of substance to the number of entities

9. How many grams of silver, Ag, are in 0.0342 mol Ag? To convert moles Ag to grams Ag, use the molar mass of Ag from the periodic table.

10. How many atoms of Ag are in 0.0342 mol Ag? To convert moles to number of atoms, use Avogadro’s number.

11. Molarity” is a concentration term. It refers to the number of moles of a substance per liter of solution . If you recall, a “mole” is the quantity of substance that contains 6.02 x 10 23 (Avogadro’s number) items. You may also recall that the atomic or formula weight of a substance , in grams, contains 1 mole (6.02 x 10 23 units) of that substance.

12. It is relatively easy to work out calculations for the number of moles of substance into the concentration term “molarity”. To calculate the molarity of a solution, simply divide the number of moles of substance it contains (solute) by the volume of the solution, in liters . One mole contains Avogadro ’ s Number (6.022 x 10 23 )

13. A mole is the amount of a substance of a system which contains as many elementary entities as there are atoms in 0.012kg (or12g) of Carbon-12 A mole is that quantity of a substance whose mass in grams is the same as its formula weight E.g. Fe55.85 Iron has an atomic mass or 55.85 g mol -1 , so one mole of iron has a mass of 55.85g

14. To dilute a solution scientists add more solvent (usually water) without the addition of more solute. This lowers the overall concentration of the solution. Review the formula M 1 V 1 = M 2 V 2 or more generally   C 1 V 1 = C 2 V 2

15. You have made 53.3 mL a stock solution of 1.5 molar solution of potassium chloride. Your laboratory protocol requires a 0.8 molar potassium chloride solution for the procedure. Prepare the 0.8 molar solution needed by dilution. M 1 V 1 = M 2 V 2   1.5 M x 53.3 mL =0 . 8 x V 2   1.5 M x 53.3 mL = 0.8 M 99.938 ml = V 2

16. You have a stock solution of 95% ethanol. Your laboratory protocol requires 25 mL a solution of 20% ethanol for the procedure. How would you prepare the 25ml 20% ethanol solution needed by dilution?   Calculate the volume of stock solution needed:   M 1 V 1 = M 2 V 2   95% x V 1 = 20% x 25 mL   V 1 = 20% x 25 mL 95% V 1 = 5.26mL  

17. Determine the amount of water to add to the solution to make the 25 mL needed.   25ml - 5.26ml = 19.74ml 3. Make the solution. X = 5.26 ml of 95% ETOH + 19.74 ml H 2 O

18. What is Molar Volume? The Molar Volume, represented by V m , is the volume occupied by one mole of a substance which can be a chemical element or a chemical compound at Standard Temperature and Pressure (STP). One mole of any gas at a particular temperature and pressure has fixed volume and known as its Molar gas volume.  

19. Where V m is the volume of the substance. The standard temperature used is 273 Kelvin or 0 o C, and standard pressure is 1 atmospheres, i.e., 760 mm Hg. Experimentally, one mole of any gas occupies a volume of 22.4 liters at STP. The equation can be expressed as 1 mole of gas at STP = 22.4 liters of gas.

20. Calculate the volume of carbon dioxide gas, CO 2 , occupied by (a) 5 moles and (b) 0.5 moles of the gas occupied at STP. Solution: a) Volume of CO 2 = number of moles of CO 2 × 22.4 L = 5 × 22.4 = 112 L b) Volume of CO 2 = number of moles of CO 2 × 22.4 L = 0.5 × 22.4 = 11.2L

21. Calculate the number of moles of ammonia gas, NH 3 , in a volume of 80 L of the gas measured at STP. Solution: Volume of gas = number of moles × 22.414 L/mol Number of moles= Volume of gas 22.414

22. No of moles= 80L 22.414 =3.6