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Brilliant_brown [7]
2 years ago
12

The starting amount of radioactive iodine-131 was 250 g. The half-life is 8 days. When Leslie measured the amount, she measured

15.625 g of radioactive iodine. How many days have elapsed between measurements
Chemistry
1 answer:
love history [14]2 years ago
3 0

Answer:

32 days

Explanation:

Half life is the amount of time it takes to get to half of the starting concentration / amount.

Starting amount = 250 g

Half life = 8 days

Amount left = 15.625 g

How many days ? That is, how many half lives....

After first Half life;

Amount left = 250 / 2 = 125 g

After second half life;

Amount left = 125 / 2 = 62.5 g

After third half life;

Amount left = 62.5 / 2 = 31.25 g

After fourth half life;

Amount left = 31.25 g / 2 = 15.625 g

That is the radioactive iodine - 131 went though four half lives.

How many days = Number of half lives * Half life

How many days = 4 * 8 = 32 days

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A solution is formed by mixing 15.2 g KOH into
nikitadnepr [17]

Answer:

There are approximately 0.271\; \rm mol of formula units in that \rm 15.2\; g of \rm KOH (the solute of this solution.)

Explanation:

A solution includes two substances: the solute and the solvent. Note the solution here contains significantly more water than \rm KOH. Hence, assume that water is the solvent (as it is in many other solutions.)

The (molar) formula mass of \rm KOH is necessary for finding the number of moles of

  • One \rm K atom,
  • One \rm O atom, and
  • One \rm  H atom.

The formula mass of \rm KOH will thus be the sum of:

  • The mass of one mole of \rm K atoms,
  • The mass of one mole of \rm O atoms, and
  • The mass of one mole of \rm H atoms.

On the other hand, the mass (in grams) of one mole of atoms of an element is (numerically) the same as its relative atomic mass. The relative atomic mass data can be found on most modern periodic tables.

Relative atomic mass data from a modern periodic table:

  • \rm K: 39.098.
  • \rm O: 15.999.
  • \rm H: 1.008.

For example, the relative atomic mass of \rm K (potassium, atomic number 19) is 39.098 (3 sig. fig.) Hence, the mass of one mole of

The formula mass of \rm KOH is the sum of these three masses:

\begin{aligned}& M(\mathrm{KOH}) \\ &\approx 39.098 + 15.999 + 1.008 \\ &= 56.105\; \rm g \cdot mol^{-1}\end{aligned}.

The number of moles of \rm KOH formula units in this 15.2\; \rm g sample would be:

\begin{aligned}n &= \frac{m(\mathrm{KOH})}{M(\mathrm{KOH})} \\ &\approx \frac{15.2\; \rm g}{56.105\; \rm g \cdot mol^{-1}} \approx 0.271\; \rm mol \end{aligned}.

6 0
2 years ago
When 6.13 g of a certain molecular compound X are dissolved in 90. g of formamide (NH2COH), the freezing point of the solution i
belka [17]

Answer:

321.6 g/Mol

Explanation:

mass of solvent in kilograms = 90g/1000 = 0.09 Kg

Given that;

ΔTf = Kf . m . i

Where;

Kf = freezing point constant = 4.25 °C/Kg mol

m = molality of the solution

i = Van't Hoff factor = 1 (since the substance is molecular)

ΔTf = freezing point of pure solvent - freezing point of solution

freezing point of pure solvent = 3 °C

ΔTf = 3 °C - 2.1 °C

ΔTf = 0.9  °C

0.9= 4.25 * 6.13/M/0.09 * 1

0.9= 26.0525/M * 1/0.09

0.9 = 26.0525/0.09 M

0.9 * 0.09M = 26.0525

M = 26.0525/0.9 * 0.09

M= 321.6 g/Mol

8 0
2 years ago
When sodium metal is added to water, an orange flame is observed on the metal surface. Based on this observation, what can best
tangare [24]

Explanation:

A property which causes change in chemical composition of a substance is known as a chemical property. For example, reactivity, toxicity, combustion etc.

Whereas a property which causes no change in chemical composition of a substance is known as a physical property. For example, mass, volume, density etc.

Sodium is a very reactive metal and it reacts readily with oxygen, water etc. So, when sodium is added to water then occurrence of an orange flame represents its reactivity.

Thus, based on given observation it can be concluded that chemical reaction takes place when sodium is added to water.

6 0
3 years ago
A 13.5 g sample of an unknown gas occupies 5.10 L at 149.83 kPa and 301 K. What is the molar mass of the gas ?
alisha [4.7K]

Answer:

The molar mass of the gas is 44.19 g/mol

Explanation:

Amount of sample of gas = m = 13.5 g

Volume occupied by the gas = V = 5.10 L

Pressure of the gas = P = 149.83 KPa

1 KPa = 0.00986 atm

P = 149.83 \textrm{ KPa} \times 0.00986 \textrm{ atm/KPa} = 1.48 \textrm{ atm}

Assuming M g/mol to be the molar mass of the gas

Assuming the gas is behaving as an ideal gas

\textrm{PV} =\textrm{nRT} \\\textrm{PV} = \displaystyle \frac{m}{M}\textrm{ RT } \\1.48 \textrm{ atm}\times 5.10 \textrm{ L} = \displaystyle \frac{13.5 \textrm{ g}}{M}\times 0.0821 \textrm{ L.atm.mol}^{-1}.K^{-1}\times 301\textrm{K} \\M = 44.19 \textrm{ g/mol}

The molar mass of gas is 44.19 g/mol

7 0
3 years ago
This is Q7 and I need help, can someone help.
andre [41]

Answer:

Answer to A. helium, neon, argon, krypton, xenon, and radon, B. Elemental hydrogen (H, element 1), nitrogen (N, element 7), oxygen (O, element 8), fluorine (F, element 9), and chlorine (Cl, element 17) are all gases at room temperature, and are found as diatomic molecules (H2, N2, O2, F2, Cl2). C. Elements Compounds

Ar (argon) HBr (hydrogen bromide) C 3H 8 (propane)

Kr (krypton) HI (hydrogen iodide) C 4H 10 (butane)

Xe (xenon) HCN (hydrogen cyanide)* CO (carbon monoxide)

Rn (radon) H 2S (hydrogen sulfide) CO 2 (carbon dioxide)

Explanation:

5 0
2 years ago
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