I’m in chem HL, and as I’ve been doing past papers for the past month, I’ve noticed trends in the way IB marks our answers. I wanted to compile everything in a long list to read before the exam started, so I thought I’d share. Some of them may be incorrect, if so, please comment! 
Disclaimer: These are not study tips or facts to memorize!

*These are general trends I’ve seen, but they may deviate from the norm, so err on the side of caution.

Calculations

• Show each subcalculation; the correct final answer gives you all the marks, but 0 if you have the wrong answer and no steps shown.
• When they give you an incorrect answer to use in a later question in case you couldn’t figure out a previous calculation, it is generally close to the actual answer. This can help you check if your previous calculation is incorrect. The number of 10’s places is generally the same. E.g. If you got 160, but they say to use 0.06 if you didn’t get the answer, your answer is likely wrong. If you got 135 and they say to use 150, you’re likely correct. *But proceed with caution!
• When doing lots of calculations with multiple steps, don’t store the number in your calculator and use the long decimal places. Just use the 3.s.f from the previous calculation or the max number of sig figs that a value they give you uses. The mark scheme uses 3 s.f., so you will get a slightly off number from the mark scheme. 
• Write 3 s.f. as a general rule, but if relevant, match your final answer to the number of sig figs of the least precise number that they give you that you use in your calculation
• You get one mark if the numbers are correct but the sign is wrong. But this depends on how many marks there are in total, and whether you showed your steps. 
• If asked to calculate the experimental value of something given in the data booklet, giving the data booklet value gives you no marks; you have to use the values they provide you with. 
• If the third sig fig is 0, just the 2 sig figs are accepted

Units related:

• Units are not required for sub-calculations for you to get the calculation mark
• Units are only required for final answers if the question asks for the answer in a specific unit
• Any units are accepted as long as they are correct. e.g. m or nm, J or kJ, unless the question asks for the answer to be in a specific unit.
• Entropy (S) is in Joules, but when used in calculations, you almost always have to convert it to kilojoules
• Check the units required for use in the formula! If unsure, multiply/divide the units together, and make sure it matches the format of the final answer's units

Written Questions

• When asked for the electron configuration, not the full configuration, the condensed form is accepted
• State symbols are generally not required if not asked for in the question. The markscheme says that it is fine, “unless directed otherwise in the ‘Notes’ column."
• When writing the oxidation state of a metal, either the number or roman numeral is accepted. If unsure if the sign goes before or after, do the Roman numeral. (e.g., +2/II, but 2+ or 2 are not accepted). As a tip, for oxidation state/number, the number comes second, but for ionic charge, the charge (+/–) comes second.
• When drawing lewis formulas, make sure to put the bracket and charge if it’s an ion
• For an O–H group, "hydroxyl" or "hydroxy" is accepted, not "alcohol" or "hydroxide."
• For an =O group, identify as a carbonyl, not a ketone or aldehyde
• When writing IUPAC nomenclature, missing hyphens/dashes are not penalized
• When predicting the product of a carbon compound, either the name or structure is accepted
• Van der Waals force is not accepted as an intermolecular force. Has to be london (dispersion) forces/dispersion forces/instantaneous/transient/induced dipole attractions
• When asked to identify the mechanism of a reaction, nucleophilic substitution reaction or correctly choosing either Sn1 or Sn2 is accepted
• When identifying bonds responsible for IR absorptions, draw the bond; don’t write the name (e.g., C=C instead of alkenes)
• In lewis formulas, any combination of dots, crosses, or lines are accepted
• Either bent / V-shaped / angular is accepted when identifying the molecular domain geometry
• When identifying bond types, either coordination/dative/coordination covalent is accepted 
• Amino/amine is accepted and primary/secondary/tertiary is not required

Let's get that 7!