Chemistry: Chapter 4

mass of 55 moles of NO2

55 MOLES x 46g

what is the *MASS* of 6.55 moles of the cmpd. CH4 ?

6.55 MOLES x 16g

? moles in 99g of CH4

99g/16g

A student needs 250.0 mL of a 0.100M CuCl2 solution. A stock solution of 2.00M CuCl2 is available

M1xV1=M2xV2 -2.00M x V1=0.100M x 0.2500L -divide 0.100M x 0.2500 by 2.00M -V1=0.0125L or 12.5mL *place 12.5 mL and dilute until it reacheas a volume of 250.0 mL*

M=?

M=moles/volume

Captive Zero

Zeros between non-zero digits -always significant

Leading Zeros

Zeros that precede the first non-zero digit -not significant

? moles in 5.0x10^23 molecules of C12H24

5.0x10^23 molecules/Avogadro's #

? moles in 333g of C6H12O6

(12x6) + (1x12) + (16x6)/180

What is the mass of 3.35x10^22 atoms of Al

(3.35x10^22/Avogadro's #) (27g)

% yield

(measure value/true value) x 100 *higher # = better*

% error

(true value- measure value)/true value

A student needs to prepare 100. mL of a 0.50M NH4Cl solution

*100 mL=.1L* .1L x 0.50M=.05 moles .05 x 53.5g

Empirical Formula of 43g sample of a cmpd -40% C, 6.7% H, 53.3% O

*C=17.2g* *H=2.881g* *O=22.919g* C-(17.2g/12=1.43333333...)/1.4324375=1 H-(2.881g/1=2.881)/1.4324375=2 O-(22.919g/16=1.4324375)/1.4324375=1 Empirical Formula: CH2O

*1000* how many significant figures?

1

? grams of NaSO4 are contained in 1.50L of 0.25M solution

1.5L x .25M= .375 moles .375 moles x 142g

Molar Mass of NO2

14 + (16x2)

which one is least precise/significant 150.0 or 0.507

150.0

? grams of Cu in 2.5 moles

2.5 MOLES x 63.55g

? molecules in 20.6 moles of the cmpd. CH4

20.6 MOLES x Avogadro's #

? moles of C in 26.0g

26.0g/12g

FeO=36g total -28g Fe, 8g O

28/36=78% Fe 8/36=22% O

*1000.* how many significant figures?

4

*2005* how many significant figures?

4

Percent Composition of C3H6O3

C-(36/90)x100 H-(6/90)x100 O-(48/90)x100

when adding/subtracting significant figures you...

make sure your answer has the same amount of #s as the least precise measurement used

Accuracy

the agreement between a measure value and a true value

Precision

the agreement between several measures

when multiplying/dividing significant figures you...

your answer is limited to the measurement with the least number of significant figures