1.) Sodium-24 has a half-life of 15 hours. How much of a 40 mg sample would rema
ID: 838362 • Letter: 1
Question
1.) Sodium-24 has a half-life of 15 hours. How much of a 40 mg sample would remain after four half-lives?
2.) Which physical state has the lowest entropy?
a.) Solids
b.) liquids
c.) gases
d.) solids and liquids have the same
e.) all have the same.
3.) The fact that a heat pump requires energy to move heat from a colder object (outside of the house) to a hotter object (inside the house) is a real-life observation of which thermodynamic law?
a.) 1st law
b.) 2nd law
c.) 3rd law
d.) 4th law
e.) none
4.) Bismuth has a mass number of 214 and an atomic number of 83 and decays by beta emission. The prodcut of this decay is?
Explanation / Answer
1-
A = A0(1/2)t/T1/2
A0 = 40 mg
t = 3 x T1/2
the remained amount of 24Na = A =40 x (1/2)3 = 40/8 = 5 mg
Second method
half life = ln (2) / decay constant ( K)
k = 0.693/15
time t = 3 * 15 = 45 hour
Lt= L0 * e ^-Kt Lt is sample left after time t
L45 = 40 mg * e ^ -( 0.693/15*45)= 40 * 0.1250= 5 mg is left
2. Solids
3. b.) 2nd law
4. SInce the atomic mass (214) remains the same but the atomic number (82) increases by one, this means that the decay occurs through an emission of a beta particle which is an electron with negligable mass (hence no change in atomic mass) and a charge of negative one (-1), hence the atomic number increases by one.
Bismuth always has atomic number of 83 so 214 Bi is more fully 214/83 Bi
Remember each alpha decay reduces atomic mass by four and atomic number by two whereas each beta decay has no effect on atomic mass but increases the atomic number by one .
Above we have:
2 alpha emissions reduce atomic mass by 8 and atomic number by 4
3 beta emissions increase atomic number by 3
Overall we have a reduction of 8 in atomic mass and a reduction of (4 - 3 =) 1 in atomic number resulting in 206/82 Pb
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