Sometimes you can just look at the atomic mass of an element on the periodic Tab

Question

Sometimes you can just look at the atomic mass of an element on the periodic Table and give a pretty good "guesstimate" of its isotopic abundance. Consider 35Cl and 37Cl. The weighed mass average of cl is 35.45. Right away you know there's going to be more 35Cl. If the weighted mass was 36 you'd know you have 50-50% mix of the two isotopes at the 35.45, you'd guess at 75% 35cl and 25% 37cl calculate exactly you would have use algebra. Let x represents the abundance of 35cl. How would you write out the algebraic expression for the weighted mass average if the exact masses of 35cl and 37 cl were respectively 34.96885 and 36.96590? Fill in the column on the left side of the table as True of False in correlations to the question on the right column of the table. All molecules move at the same speed at STP. Gaseous water particles weigh less than solid water particles with the same number Positive atoms have less electrons than protons. The atomic number, X, is the number of neutrons. Avogadro's Law states that 1 mole of liquid at 1.0 torr pressure, and at 273.15 degree K temperature will occupies a volume of 22.4 L Isotopes have the same number of protons but differing number of electrons. Leveling the bulb in the gasometrical experiment was used to keep constant volume. Reactions that releases heat is called an exothermic reaction.

Explanation / Answer

8. The abundance of 35Cl = x

Then abundance of 37Cl = (1-x)

The atomic mass of Cl = 35.45

The exact mass of 35Cl = 34.96885

The exact mass of 37Cl = 36.96590

Atomic mass = abundance of 35Cl + abundance of 37 Cl

The algabraic equation will be:

35.45 = (34.96885x) + [36.96590(1-x)]

If you want to solve x, then,

35.45 = (34.96885x) + [36.96590-(36.96590x)]

-1.99705x = -1.5159

x = 75.91%

Abundance of 35Cl = 0.7591 = 75.91 %

Abundance of 37Cl = 1-0.7591 = 0.2409 = 24.09 %

9. (i) All molecules travel at same speed at STP: The speed depends on temperature and molar mass of the gas molecules. At STP, all gases are at same temperature. But they have different molar masses. So, the statement is false.

(ii) Gaseous water molecules weigh less than solid water molecules: If the number of particles is same, then it means that the number of moles is same. This means that equal mass of gaseous and solid are preseant. So, they should weigh same. This statement is also false.

(iii) Positive Atoms have less electrons than protons: the positive atoms means cations, which are formed by removing the electrons. Thus electrons will be less than protons. This statement is true.

(iv) The atomic number Z is the number of neutrons: The Z is the number of electrons or number of protons. It is not number of neutrons. The statement is false.

(v) Avogadro's law is for gases. So, this statement is also false.

(vi) Isotopes are same number of protons, but different number of neutrons. The statement is false.

(vii) Leveling the bulb was for constant pressure. The statement is false.

(viii) The statement is true. Exothermic reactions releases heat and endothermic reaction uses heat.

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