1. Elements vs Compound Elements –First letter of element symbol is always capitalized –Only have one capitalized letter Compounds –Combination of elements –Can recognize by how many capital letters (2 or more) H 2 ONaClFe O2Ba(OH) 2 K

2. Regardless of amount of compound we have, the compound will always be comprised of the same elements in the same proportions by mass % by mass = mass of element mass of compound Law of Definite Proportions x 100

3. 20 grams from a sugar bag ElementMass% by Mass C 8.4 g (8.4 / 20.0 ) * 100 = 42% H 1.3 g (1.3 / 20.0) * 100 = 6.5% O 10.3 g (10.3/20.0) * 100 = 51.5% Total20.0 g 100%

4. 500 grams from Sugar Cane ElementMass% by Mass C 210.5 g (210.5 / 500.0 ) * 100 = 42.1% H 32.4 g (32.4 / 500.0) * 100 = 6.48% O 257.1 g (257.1/ 500.0) * 100 = 51.4% Total500.0 g 100%

5. Let’s Compare % by Mass CarbonHydrogenOxygen 20 grams of sugar from a bag 42%6.5%51.5% 500 grams of sugar from sugar cane 42.1%6.48%51.4% Are they the same compound????

6. % by Mass CarbonHydrogenOxygen 20 g from a bag 42%6.5%51.5% 500 g from sugar cane 42.1%6.48%51.4% Compounds with same mass proportions  must be the same compound Compounds with different mass proportions  must be different compounds

7. 1. If hydrogen composes 11% of a 100 gram sample of water, what is the mass of the hydrogen in the sample? Law of Definite Proportions Practice I

8. 2. If one gram of hydrogen reacts completely with 19. g of fluorine, what is the percent by mass of hydrogen in the compound that is formed?

9. 3. If 3% of a 100 g HCl solution is hydrogen, what is the mass of chlorine in the sample?

10. 4. A 78.0 g sample of an unknown compound contains 12.4 g of hydrogen. What is the percent by mass of hydrogen in the compound?

11. 5. If 40% of a 300 g NaCl solution is sodium, what is the mass of the sodium present in the sample?

12. 1. Sodium has an atomic mass of 23 amu’s and chlorine has an atomic mass of 25 amu’s. In the compound NaCl, what is the % composition by mass of sodium? Law of Definite Proportions Practice II

13. 2. Sulfur composes 33% of the compound H 2 SO 4. What would be the mass of sulfur in a 500 gram sample of H 2 SO 4 ?

14. 3. If 32 grams of sulfur reacts completely with 32 grams of oxygen, what is the mass of sulfur in 800 g of SO 2 ?

15. 4. An 800 g sample of an unknown substance contains 121 g of hydrogen. What is the % composition of hydrogen by mass in the substance?

16. 5. If 28.8% of a 600 gram sample of NaBr is sodium, what is the mass of the sodium present in the sample?

17. How many grams of sodium are contained in a 500 gram sample of sodium chloride (NaCl)? Na = 23.0  periodic Table Cl = 35.4  periodic Table Total= Na + Cl = 23.0 + 35.4 = 58.4 g % Na = (23 / 58.4) * 100 = 39.38% 500 g * 39.38% = 500 * (39.38/100) = 197 g

18. 1. Sodium has an atomic mass of 23 amu’s and chlorine has an atomic mass of 25 amu’s. In the compound NaCl, what is the % composition by mass of sodium?