Answer:
20.0928.
Explanation:
The average atomic mass is (90 * 19.992 + 10* 21) / 100
= 20.0928.
Answer:
change in the total mass of substances
A. The radioactive decay equation is N = N0
where T is the
half-life (5730 years), N0 is the number of atoms at time t = 0 and
N is the number at time t.
Rewriting this as:
(N/N0) = 
Since N = (1/8) N0 and
substituting known values:
1/8 = 
Taking ln of both
sides:
ln(1/8)= -ln(2)*t/5730
t = - 5730 * ln(1/8) /
ln (2)
t = 17,190 years
The tree was cut down 17,190
years ago.
B. N0 = 1,500,000 carbon-14 atoms
Since N = (1/8) N0
N = 187,500 carbon
atoms left
Answer:

Explanation:
Hello there!
In this case, according to the given information, it turns out firstly necessary to write out the described chemical reaction as shown below:

Now, we set up the expression for the calculation of the standard free energy change, considering the free energy of formation of each species, specially those of H2 and F2 which are both 0 because they are pure elements:

Regards!
Answer is: <span>concentration of fluoride in the water in parts-per-million is 1 ppm.
</span>Parts-per-million (10⁻⁶) is<span> present at one-millionth of a </span>gram per gram of sample solution, f<span>or example mg/kg.
</span>m(fluoride) = 500 g · 1000 mg/g = 500000 mg.
m(water) = d(water) · V(water).
m(water) = 1 kg/L · 500000 L.
m(water) = 500000 kg.
arts-per-million = 500000 mg ÷ 500000 kg = 1 mg/kg = 1 ppm.