relative molecular mass

Chemical terminology

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Relative molecular mass refers to chemical formula In atom Of Relative atomic mass （A r ）Of the sum , using symbols M r Indicates that the unit is 1.

about polymer Its relative molecular weight can reach tens of thousands or even hundreds of thousands; Smallest relative molecular weight oxide The chemical formula of is H ₂ O。

Chinese name: relative molecular mass
Foreign name: Relative molecular mass

Alias: Chemical formula quantity 、 Relative molecular weight
Unit: one
Symbol: Mr

catalog

five Common relative atomic mass

Conceptual background

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since element The relative atomic mass of is a relative mass with the unit of "1", so the molecular mass calculated from this must also be a relative mass with the unit of "1". For some biomacromolecules with complex structure, their approximate molecular weight is often measured by electrophoresis, centrifugation or chromatography, so it is a relative concept. Therefore, the "molecular weight" we used to use for a long time is actually the relative molecular weight. Therefore, the national standard points out that "formerly known as molecular weight" is "relative molecular mass", and defines the latter as "the average mass and nuclide of the molecule or specific unit of a substance" ¹² C atom quality 1/12 ".

The relative molecular weight is the ratio of two masses, which further clarifies the meaning of "relative" in the form of calculation expression. The "specific unit" in the definition mainly refers to atmosphere For special mixtures whose components are basically unchanged, their relative mass can be calculated according to the relative molecular mass of their components (N ₂, O ₂, CO ₂, Ar, etc.) and their volume fraction in air, and then compared with ¹² 1/12 of the atomic mass of C can be obtained. The weight symbol of the relative molecular weight is M r ., the unit is "1".

computing method

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with carbon dioxide Take an example to illustrate the calculation method of relative molecular weight:^[1]

Preparation, purification, drying and collection of carbon dioxide

Assemble the carbon dioxide gas generation and purification device as shown in the Diagram of Carbon Dioxide Preparation, Purification, Drying and Collection Device Start up generator CO generated by reaction in two Gas, water passing through gas washer 2, concentrated H of gas washer 3 two SO four Anhydrous CaCl in washing and drying tube 4 of two Drying or 1mol L -1 CuSO four Solution, 1mol L -1 NaHCO three Solution washing and anhydrous CaCl two After drying, the gas introduced into conical flask 5 is pure CO two Gas.^[2]

Schematic diagram of carbon dioxide preparation, purification, drying and collection device

weigh

1.m one : quality of bottle+stopper+air

Take a clean and dry conical bottle, select a suitable rubber stopper to close the mouth of the bottle, make a mark on the stopper, mark the position of the stopper in the bottle Electronic balance Upper weighing mass, recorded as m one 。

2.m two ：CO two +Mass of bottle+stopper

The carbon dioxide gas generated from the Kip generator needs to be washed, dried, collected and weighed. Because the density of carbon dioxide is greater than that of air, the catheter must be inserted into the bottom of the bottle to exhaust the air in the bottle. After the gas is fully collected; Slowly take out the catheter, plug the mouth of the bottle with a stopper (the position of the stopper inserted into the mouth of the bottle should be the same as that in step 1), and weigh the mass on the electronic balance, which is recorded as m two 。 Repeat the operation for three times until the mass difference between the three times is no more than 1mg.

3.m three : mass of water+bottle+stopper

Finally, fill the bottle with water, plug the plug, and weigh the mass on the platform scale, which is recorded as m three 。

4. Record the room temperature and atmospheric pressure^[2] 。

data processing

Data filling and calculation shall be carried out in accordance with the following figure: Determination Data Record and Result Processing of Carbon Dioxide Relative Molecular Weight^[2]

Data recording and result processing of carbon dioxide relative molecular weight measurement

Calculation format

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M r =A r +Arasz

For example, ethanol (C ₂ H ≮ OH):

matters needing attention

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The relative molecular mass is numerically equal to the molar mass, but in different units. The unit of relative molecular weight is "1", while the unit of molar mass is g/mol; And the oxide with the smallest relative molecular weight chemical formula Is H ₂ O.

Common relative atomic mass

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Element Name	Element symbol	Relative atomic mass
hydrogen	H	one
carbon	C	twelve
nitrogen	N	fourteen
oxygen	O	sixteen
sodium	Na	twenty-three
magnesium	Mg	twenty-four
aluminum	Al	twenty-seven
silicon	Si	twenty-eight
phosphorus	P	thirty-one
sulfur	S	thirty-two
chlorine	Cl	thirty-five point five
potassium	K	thirty-nine
calcium	Ca	forty
manganese	Mn	fifty-five
iron	Fe	fifty-six
copper	Cu	sixty-four
zinc	Zn	sixty-five
silver	Ag	one hundred and eight
iodine	I	one hundred and twenty-seven
barium	Ba	one hundred and thirty-seven
gold	Au	one hundred and ninety-seven

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