FORMULAE AND CHEMICAL EQUATIONS INTRODUCTION RELATIVE ATOMIC RELATIVE MOLECULAR MASS (RAM) MASS (RMM)EXERCISE CARBON-12 SCALE EXERCISE
INTRODUCTION Mass of one atom is small. For example the mass of one atom cuprum 1.06 x10-22 gram. We can’t weigh one atom directly but we can compare the mass of one atom with another mass of one atom by using the relative mass scale.
RELATIVE ATOMIC MASS (RAM) DefinitionThen, relative atomic mass ofelement:= mass of one element 1/12 x mass of one atom C-12 Example
DEFINITION OF RELATIVE ATOMIC MASS (RAM) RELATIVE ATOMIC MASS (RAM) of elements is how much time one atom of element more heavy than 1/12 mass of one atom C-12 with the mass of one atom C-12 is 12.000.
EXAMPLE: The mass of one atom potassium is 39 time more bigger than 1/12 x mass of one atom C-12. So, relative atomic mass for one atom potassium is 39. K = 39
RELATIVE MOLECULAR MASS (RMM) Definition Relative molecular mass of compound= Mass of one molecule of compound 1/12 x mass of one atom C-12 Example
DEFINITION OF RELATIVE MOLECULAR MASS (RMM) RELATIVE MOLECULAR MASS (RMM) is how much time the mass of one molecule is compound is more bigger than 1/12 mass of one atom C-12 with the mass of one atom C-12 is 12.000.
EXAMPLE: Relative molecular mass for water, H2O: = 2 x (relative atomic mass H) + 1 x (relative atomic mass O) = ( 2 x 1) + ( 1 x 16) =18So, relative molecular mass for H2O is 18. (RAM: H = 1; O = 16)
EXAMPLE: Relative molecular mass for HCl. = 1 x (relative atomic mass H) + 1 x (relative atomic mass Cl) = (1 x 1) + (1 x 35.5) = 36.5 So, relative molecular mass for HCl is 36.5. (RAM : H=1 ; Cl=35.5 )
INTRODUCTION OF CARBON-12 At the beginning, all chemists agreed to use atom hydrogen as a standard in determined RAM and RMM with one atom hydrogen is given as 1 units. Then, chemists found that this standard is not suitable because:
In 1860, chemists agreed to use atom oxygen as a standard. This is because: A problem occurred when there are two standard of relative atomic mass. Finally, in 1961 all scientists agreed to use isotope 126C as a standard. This is because:
WHY ATOM HYDROGEN IS NOT SUITABLE AS A STANDARD: Hydrogen has isotopes, which is 11H, 21H (deuterium) and 31H (tritium). So, hydrogen is not 1 units. Hydrogen existed as gases and its masses was very difficult to determine. There are a lot of elements that is not react with hydrogen.
WHY ATOM OXYGEN USED AS A STANDARD: Atom oxygen can react with many elements. Most of the compounds produced oxide is existed as solid. The mass of this compound was easy to determine.
WHY CARBON-12 IS USED AS A STANDARD FOR DETERMINING RAM AND RMM Carbon existed is many kinds of materials and easy to get. Carbon occurred as a solid in room temperature compare with oxygen gas. This element doesn’t need a stopper to keep it. The existing of other isotope such as 126 C,136 C and 146 C are too little.
WHAT IS THE RELATIVE ATOMIC MASS OF THESE ELEMENTS?? C =? Na = ? O =? Al = ? N =? Fe = ? F =? Ar = ? Br =? Mn = ?
C = 12 O = 16 N = 14 F = 19 Br = 80 Na = 23 Al = 27 Fe = 56 Ar = 40 Mn= 55
WHAT ARE THE RELATIVE MOLECULAR MASSES OF THESE MOLECULES?? NaCl ? H2S ? H2O ? CaCl2 ? HCl ? K2O ? CuO ? NaOH ? MgCl2 ? CO2 ?(RAM: H=1; C=12; O=16; Na=23; Mg=24; S=32; Cl=35.5; K=39; Ca=40; Cu=64)