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Questions and Answers
What is the symbol of the SI unit of luminous intensity in a given direction?
The kilogram is defined in terms of the speed of light.
True
What is the numerical value of the luminous efficacy of monochromatic radiation of frequency 540×10^12 Hz?
683
The unit of luminous intensity is defined in terms of the kilogram, metre, and _______________.
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Match the following physical constants with their descriptions:
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What is the frequency of the monochromatic radiation used to define the luminous efficacy?
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What is the relative abundance of the isotope 12C?
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The atomic mass mentioned in the periodic table of elements represents the molecular mass of an element.
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What is the formula used to calculate the average atomic mass of an element?
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The average atomic mass of carbon is approximately ______________ u.
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What is the reason for using the formula mass instead of molecular mass in the case of sodium chloride?
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The atomic mass of 13C is exactly 13 u.
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Match the following isotopes of carbon with their relative abundances:
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In sodium chloride, each Na+ ion is surrounded by ______________ Cl– ions.
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What is the purpose of prefixes in the SI system?
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The speed of light in vacuum is defined as 299792458 ms–1.
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What is the symbol for the SI unit of length?
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The kilogram is defined by taking the fixed numerical value of the ______________________ constant h.
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What is the SI unit of electric current?
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The second is defined in terms of the unperturbed ground-state hyperfine transition frequency of the caesium-133 atom.
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Match the following SI base units with their definitions:
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What is the value of the caesium frequency ΔV Cs when expressed in the unit Hz?
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Study Notes
Units and Measurement
- Elementary entities can be atoms, molecules, ions, electrons, or specified groups of particles.
- The candela (cd) is the SI unit of luminous intensity in a given direction.
- The candela is defined by taking the fixed numerical value of the luminous efficacy of monochromatic radiation of frequency 540×10^12 Hz to be 683 when expressed in the unit lm·W–1.
SI Base Units
- The metre (m) is the SI unit of length, defined by taking the fixed numerical value of the speed of light in vacuum (c) to be 299792458 when expressed in the unit ms–1.
- The kilogram (kg) is the SI unit of mass, defined by taking the fixed numerical value of the planck constant (h) to be 6.62607015×10–34 when expressed in the unit Js.
- The second (s) is the SI unit of time, defined by taking the fixed numerical value of the caesium frequency ΔV Cs to be 9192631770 when expressed in the unit Hz.
- The ampere (A) is the SI unit of electric current.
Isotopes and Atomic Mass
- Carbon has three isotopes with relative abundances and masses: 12C (98.892%, 12 amu), 13C (1.108%, 13.00335 amu), and 14C (2 × 10–10, 14.00317 amu).
- The average atomic mass of carbon is calculated to be 12.011 u.
- Average atomic masses for other elements can be calculated similarly.
- In the periodic table of elements, the atomic masses mentioned represent their average atomic masses.
- In the solid state, sodium chloride (NaCl) does not exist as a single entity, and the formula mass is calculated instead of molecular mass.
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