Molar Mass Calculating the molar mass of Compounds.

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Presentation transcript:

Molar Mass Calculating the molar mass of Compounds

1. Molar Mass The mass in grams of ONE mole of the element, compound, or molecule The mass in grams of ONE mole of the element, compound, or molecule The mole allows us to talk about how much we have of a substance in reasonable units (grams) rather than extremely small units (amus) The mole allows us to talk about how much we have of a substance in reasonable units (grams) rather than extremely small units (amus)

2. Units of Molar Mass UNITS OF MOLAR MASS = GRAMS/ MOLE

What is mass of one mole of….. What is mass of one mole of….. a. O atoms? b. Cl atoms? c. K atoms? c. K atoms? 3. Molar Mass for each element Find it on the Periodic Table

We can use the molar mass of an element to figure out the mass of a given number of moles for that element. Start with the NUMBER AND UNITS GIVEN. Use the molar mass as a conversion factor, and cancel out units in the denominator to leave units you want in the numerator. a. Determine the mass of 5 moles of N atoms: a. Determine the mass of 5 moles of N atoms: 5 moles N x _____________ = grams 5 moles N x _____________ = grams b. Determine the mass of 0.2 moles of C atoms: c. Determine the mass of 1.3 moles of Na atoms: c. Determine the mass of 1.3 moles of Na atoms: 4. Moles to Mass

The molar mass can ALSO be used as a conversion factor to find out how many moles are in a given number of grams of an element. Be careful which units you write in the numerator and the denominator. Remember, the units you want to cancel should be in the denominator. a. Calculate the moles in 30 grams of Mg atoms: 30 grams Mg x ___________ = moles b. Calculate the number of moles in 7.5 grams of O atoms: c. Calculate the number of moles in 2.54 grams of Cl atoms: 5. Mass to Moles

Using Avogadro’s number as a conversion factor, we can also figure out how MANY atoms we have in a certain mass of an element or the mass of a certain number of atoms. a. Avogadro’s number = _________________ a. Avogadro’s number = _________________ b. Calculate the number of Br atoms there are in 100 grams of Br. b. Calculate the number of Br atoms there are in 100 grams of Br. 100 grams x _______ x ___________ = ___atoms 100 grams x _______ x ___________ = ___atoms c. Calculate the number of F atoms in 12.6 grams of F. c. Calculate the number of F atoms in 12.6 grams of F. d. Calculate the number of C atoms in 4.2 grams of C. d. Calculate the number of C atoms in 4.2 grams of C. e. Calculate the mass of 2.6 x 1024 atoms of Cl. e. Calculate the mass of 2.6 x 1024 atoms of Cl. f. Calculate the mass of 4.3 x 1022 atoms of Na. f. Calculate the mass of 4.3 x 1022 atoms of Na. 6. Using Avogadro’s number

Now calculating molar mass of compounds

Calculating molar mass for a compound: In order to calculate the molar mass of a compound, we have to consider how many atoms of each element are in that compound. Example: Calculate the molar mass of NaCl. BE SURE TO INCLUDE UNITS!!! First, find the mass of each element in one mole of compound: First, find the mass of each element in one mole of compound: Number of atoms of Element A in compound x Molar Mass Element A Number of atoms of Element A in compound x Molar Mass Element A a. How many moles Na atoms are in one mole of the NaCl compound? a. How many moles Na atoms are in one mole of the NaCl compound? b. Mass of Na in one mole of the compound: b. Mass of Na in one mole of the compound: c. How many moles of Cl atoms are in one mole of the NaCl compound? c. How many moles of Cl atoms are in one mole of the NaCl compound? d. Mass of Cl in one mole of the compound: d. Mass of Cl in one mole of the compound:

ANSWERS a. How many Na atoms are in one NaCl compound? a. How many Na atoms are in one NaCl compound? 1 Na atom per NaCl (aka one mole Na in one mole NaCl) b. Mass of Na in the compound: b. Mass of Na in the compound: 1 mole Na x 23 grams Na = 23 grams 1 mole 1 mole c. How many Cl atoms are in one NaCl compound? c. How many Cl atoms are in one NaCl compound? 1 Cl atom per NaCl (aka one mole Cl in one mole NaCl) d. Mass of Cl in the compound: d. Mass of Cl in the compound: 1 mole Cl x grams = grams 1 mole 1 mole

Molar mass NaCl Second, add the mass of each element in the compound together: Second, add the mass of each element in the compound together: Mass of compound A + Mass of compound B = Molar Mass of AB Mass of compound A + Mass of compound B = Molar Mass of AB Mass Na + Mass Cl in one mole NaCl = Mass Na + Mass Cl in one mole NaCl =

Molar Mass NaCl 23 GRAMS GRAMS = GRAMS 23 GRAMS GRAMS = GRAMS

5. Calculate the molar mass of N 2 O. INCLUDE UNITS!! N: N: # moles N atoms in 1 mole X MOLAR MASS N O: # moles O atoms in 1 mole X MOLAR MASS O Mass N + Mass O = Mass N + Mass O =

Answers N: N: moles N atoms in 1 mole X MOLAR MASS N 2 x 14 grams/mole = 28 grams N O:O:O:O: Moles O atoms in 1 mole X MOLAR MASS O 1 x 16 grams/mole = 16 g O Mass N + Mass O = 44 grams Mass N + Mass O = 44 grams

Practice 1. Calculate the molar mass of dichlorine heptoxide: Cl:O: + _____________

Answer Cl:2 x = 70.9 Cl:2 x = 70.9 O: 7 X 16= 112 O: 7 X 16= ___________ 183 grams/mole

Calculate the molar mass of Mg(ClO 3 ) 2 Mg:Cl:O: + ____________

Answer Mg: 2 x 24.3= 48.6 grams Mg: 2 x 24.3= 48.6 grams Cl:2 x = 70.9 grams Cl:2 x = 70.9 grams O: 6 x 16 = 96 grams O: 6 x 16 = 96 grams + _________ + _________ grams/mole grams/mole

Calculate the molar mass of Ca(NO 3 ) 2 Ca: Ca: N: N: O: O:

Answer Ca: 1 x 40.1= 40.1 Ca: 1 x 40.1= 40.1 N:2 X 14= 28 N:2 X 14= 28 O:6 X 16= 96 O:6 X 16= 96 +____________ +____________ grams/mole